Meanwhile, German Chancellor Olaf Scholz will visit Ukraine on Monday and Russia on Tuesday in an attempt to calm down the prospects of invasion through diplomatic talk.
Ukrainian President Volodymyr Zelensky spoke on the phone to his US counterpart Joe Biden on Sunday about the crisis at its border.
The two leaders agreed to pursue "diplomacy" and "deterrence" against Russia in a telephone conversation lasting about 50 minutes, the White House reported.
"The two leaders agreed on the importance of pursuing diplomacy and deterrence in response to the Russian military buildup on Ukraine's borders," the executive summary statement said.
During the exchange, Biden again promised a "swift and decisive" US response, in coordination with its allies, in the event of a Russian attack.
Earlier this weekend, Zelenskyy questioned the warnings from US officials in recent days that Russia could be planning to invade as soon as midweek, saying he had yet to see convincing evidence.
“We understand all the risks, we understand that there are risks," he said in a live broadcast. "If you, or anyone else, has additional information regarding a 100% Russian invasion starting on the 16th, please forward that information to us.”
Meanwhile, German Chancellor Olaf Scholz is flying to Ukraine and Russia this week in an effort to help defuse escalating tensions as Western intelligence officials warn that the possibility of a Russian invasion of Ukraine is increasing.
Adding to tensions, Russia launched new naval manoeuvres in the Black Sea on Saturday to practice "defending" Crimea, the peninsula it annexed from Ukraine in 2014.
On Saturday, the defence ministry also claimed that the Russian navy had driven a US submarine from its waters in the Pacific Ocean.
The Pentagon stated "there is no truth" to Moscow's claims about US operations in Russian territorial waters.
Russia has concentrated more than 100,000 troops near Ukraine’s borders and launched a series of military manoeuvres in the region but says it has no plans to invade the nation.
Moscow wants guarantees from the West that NATO will not allow Ukraine and other former Soviet countries to join as members and for the alliance to halt weapon deployments to Ukraine and roll back its forces from Eastern Europe. The US and NATO flatly reject these demands.
What is Scholz's plan?
Ahead of his first visits as chancellor to Kyiv on Monday and Moscow on Tuesday for meetings with the Ukrainian and Russian presidents, Scholz renewed his warning to Russia and his advocacy of continuing diplomacy in multiple formats.
“It is our job to ensure that we prevent a war in Europe, in that we send a clear message to Russia that any military aggression would have consequences that would be very high [...] and that we are united with our allies,” Scholz told the German parliament’s upper house on Friday.
“But at the same time that also includes using all opportunities for talks and further development,” Scholz said.
Scholz has repeatedly said that Moscow would pay a “high price” in the event of an attack. Still, his government’s refusal to supply lethal weapons to Ukraine or spell out which sanctions it would support against Russia have drawn criticism abroad and at home and raised questions about Berlin’s resolve to stand up to Russia.
Asked on Friday whether Scholz will be taking any new initiative to Kyiv and Moscow or the positions that are already on the table, his spokesman, Steffen Hebestreit, replied that he would stick with “the positions that we have already set out”.
The Ukrainian ambassador to Berlin, however, decried what he called "German hypocrisy" on Sunday.
Andrij Melnyk wrote on Twitter that while Berlin is not sending weapons "for Ukraine's self-defence against Russia military invasion", it has exported 366 million worth of dual-use goods in Russia in 2020 "which can be destined to boost weapons production".
EU sanctions imposed in 2014 following Russia's annexation of Crimea ban member states from importing or exporting defence-related materials as well as dual-use goods for military use.
Germany’s reluctant position is partly rooted in its history of aggression during the 20th century when the country's militarisation in Europe during two world wars led many postwar German leaders to view any military response as a very last resort.
Experts say despite this historical burden, it is of utmost importance now that Scholz stresses Germany is in sync with its European and American allies, especially when he meets with Russian President Vladimir Putin.
“Scholz has to convey a very clear message in Moscow, and it can really only be: There is unity and oneness in the Western alliance," said Markus Ziener, an expert with the German Marshall Fund.
“At the same time, he has to make it clear that the costs are high,” Ziener added. “That’s basically the message that is most likely to catch on in Moscow as well. So a military invasion of Ukraine has significant consequences for Russia.”
Scholz has not explicitly said what consequences or sanctions Russia would face if it invades Ukraine. Still, it is clear that the future of the controversial Nord Stream 2 pipeline that seeks to bring Russian natural gas to Germany under the Baltic Sea, bypassing Ukraine, is at stake.
US President Joe Biden threatened last week that the pipeline would be blocked in the case of an invasion.
That would hurt Russia economically but also cause supply problems for Germany. The pipeline has been completed, but it is not yet operating.
“Germany is very susceptible to blackmail. We can’t do too much," Claudia Kemfert, the head of the department of energy, transport and environment at the German Institute for Economic Research, said.
"We have committed ourselves to getting the gas supplies, unlike other European countries we have not diversified our gas supplies and we have dragged our feet on the energy transition. So we did a lot of things wrong, and now we are paying the price."
Ukrainian leadership defiant
Ukrainian President Volodymyr Zelenskyy spoke of the need for calm as he observed military exercises Saturday near Crimea, the peninsula that Russia seized from Ukraine in 2014.
"We are not afraid, we're without panic, all is under control,” he said.
The US and numerous Western governments have urged their citizens to leave Ukraine, and Washington on Saturday said it was ordering most of its embassy staff in Kyiv to leave. Canada has also shuttered its embassy in the country.
But Zelenskyy pushed back against what he labelled as excessive amounts of information about impending war.
"The best friend of our enemies is panic in our country. And all this information is just provoking panic and can't help us," he said.
"I can't agree or disagree with what hasn't happened yet. So far, there is no full-scale war in Ukraine."
"We have to be ready each day. It did not begin yesterday. It began in 2014, so we are ready and this is why we are here," Zelenskyy said, referring to Russia's annexation of Crimea and backing for an anti-Kyiv separatist insurgency in the east.
In a separate statement, the head of the Ukrainian armed forces Lieutenant-General Valery Zaluzhny and Defence Minister Oleksii Reznikov said any invader would not take Kyiv, Odessa, Kharkiv or any other city in Ukraine.
"We have strengthened the defence of Kyiv. We have gone through the war and due preparation."
“We are ready to meet the enemy, and not with flowers, but with Stingers, Javelins and NLAWs” — anti-tank and -aircraft weapons, they said. “Welcome to hell!”
In Kyiv, thousands of demonstrators marched on Saturday, saying they refused to give in to panic, even if they took the threat seriously.
"Panic is useless. We must unite and fight for our independence," said student Maria Shcherbenko, holding a sign reading "I remain calm. I love Ukraine".
KLM flights to Ukraine halted as embassies close
Dutch airline KLM has cancelled flights to Ukraine until further notice, the company said Saturday.
Dutch sensitivity to potential danger in Ukrainian airspace is high in the wake of the 2014 shooting down of a Malaysian airliner over an area of eastern Ukraine held by Russia-backed rebels.
All 298 people aboard died, including 198 Dutch citizens.
The Ukrainian charter airline SkyUp said Sunday that its flight from Madeira, Portugal, to Kyiv was diverted to the Moldovan capital Chisinau after the plane's Irish lessor said it was banning flights in Ukrainian airspace.
In a sign that US officials are getting ready for a worst-case scenario, the United States announced plans to evacuate most of its staff from the embassy in the Ukrainian capital and urged all US citizens in Ukraine to leave the country immediately.
Canada has shuttered its embassy in Kyiv and relocated its diplomatic staff to a temporary office in Lviv, located in the western part of the country, Foreign Affairs Minister Melanie Joly said Saturday.
Lviv is home to a Ukrainian military base that has served as the main hub for Canada's 200-soldier training mission in the former Soviet country.
Submarine allegations appear, Pentagon sends more troops
Further US-Russia tensions arose on Saturday when the Defense Ministry summoned the US military attache after it said the navy detected an American submarine in its territorial waters near the Kuril Islands in the Pacific.
The submarine declined orders to leave, but departed after the navy used unspecified "appropriate means," the ministry said.
Adding to the sense of crisis, the Pentagon ordered an additional 3,000 US troops to Poland to reassure allies.
In addition to the more than 100,000 ground troops that US officials say Russia has assembled along Ukraine's eastern and southern borders, the Russians have deployed missile, air, naval and special operations forces, as well as supplies to sustain a war.
This week, Russia moved six amphibious assault ships into the Black Sea, augmenting its capability to land marines on the coast.
Biden has bolstered the US military presence in Europe as reassurance to allies on NATO's eastern flank. The 3,000 additional soldiers ordered to Poland come on top of 1,700 who are on their way there.
The US Army also is shifting 1,000 soldiers from Germany to Romania, which like Poland shares a border with Ukraine.
Orbán laments potential refugee crisis
Hungary's nationalist prime minister warned Saturday that a Russian invasion of Ukraine could send hundreds of thousands of Ukrainian refugees fleeing across the border into his country.
Right-wing populist leader Viktor Orbán, speaking in an annual address that this year kicked off his political campaign for Hungary's parliamentary election on 3 April, urged a peaceful resolution to the rising tensions in Europe that have stemmed from fears of an imminent Russian invasion of Ukraine.
Orbán — a firm opponent of any immigration — said it was in Hungary's best interest to “avoid war,” which he said would cause a wave of Ukrainian refugees and a disruption of the economy.
While urging a resolution of the tensions through dialogue, Orbán said he opposed plans by the European Union to use sanctions against Russia — which has built up over 100,000 troops along Ukraine's borders — as a deterrent.
“Sanctions, punitive policies, lecturing or any other kind of arrogance on the part of the great powers are out of the question,” Orbán said.
Orbán, who has led Hungary since 2010, has one of the closest relations with Russian President Vladimir Putin of any European leader.
In a meeting with Putin in the Kremlin last week, Orbán lobbied for increased gas shipments from Russia and lauded his country's increased cooperation with Moscow in the areas of energy, trade and security.
Since Russia annexed Ukraine's Crimean Peninsula in 2014, Hungary under Orbán has consistently opposed levelling EU sanctions against Moscow, although it has always ultimately voted for them.
Hungary has also blocked ministerial meetings between Ukraine and NATO over a Ukrainian language law that Budapest argues violates the rights of the Hungarian ethnic minority in western Ukraine.
Yet on Saturday, Orbán said that Ukraine serves as a crucial buffer zone between Hungary and Russia and that its "independence and viability are therefore of direct Hungarian interest."
Declaring that "the military strength of Europe should at least be comparable to that of Russia," Orbán said that Hungary supports the development of European military capabilities and a joint European defence force.
Hungary has declined to accept military reinforcements from NATO and the United States — which have been mobilized in several other Eastern European countries in response to the buildup of Russian troops along Ukraine's borders. He says Hungary's domestic military is sufficient to protect the country.