Ursula von der Leyen has described the influx of migrants on Belarus' borders with EU countries as a "hybrid attack" by an authoritarian regime on its neighbours.
Speaking after meeting US President Joe Biden in Washington, the European Commission president vowed to "protect democracies" and said the two leaders shared a common assessment of the situation.
Threatening sanctions against countries found to be aiding Belarusian leader Alexander Lukashenko's efforts to damage the EU, von der Leyen said efforts were being made to persuade them not to "fall into the trap".
The EU has accused Belarus of organising the transport of people from Middle Eastern countries, duping them into believing they can enter Europe, as part of a campaign to create instability with a new wave of mass migration to the bloc.
"This is a hybrid attack. Not a migration crisis," the Commission chief said on Twitter.
Several thousand migrants and refugees have set up makeshift camps at Poland's border with Belarus, in freezing conditions. On Wednesday the UN High Commissioner for Refugees, Michelle Bachelet, condemned an "intolerable situation" and demanded they get "immediate" access to humanitarian aid.
Poland's prime minister accused Belarus of "state terrorism" over its role in the influx of migrants gathered at the border. Mateusz Morawiecki made the comments during a joint news conference with European Council President Charles Michel in Warsaw.
He also claimed that Lukashenko was acting out of "discreet vengeance" in response to Polish support for the opposition in Belarus. A day earlier, Morawiecki accused Vladimir Putin of orchestrating the migrant influx.
Belarus and Russia have again hit back at such accusations, Minsk accusing the EU and the West of "provoking" the border standoff.
As the crisis continued to take on an increasingly international dimension, German Chancellor Angela Merkel urged the Russian president to intervene.
'600 attempted crossings'
Polish authorities said on Wednesday that security forces had thwarted nearly 600 attempts to enter the country in 24 hours.
A Border Guard spokesperson said nine migrants had been detained — five Lebanese nationals, three Iraqis and one Syrian — and another 48 had been issued with orders to leave the country.
Polish broadcaster TVN showed footage of migrants, many of them small children, being escorted away from a forest near Narewka in Podlasie in eastern Poland.
The authorities said earlier that more than 50 people had been detained after two separate groups crossed the border illegally and entered Polish territory.
Poland's defence minister said on Wednesday that small groups were still trying to cross the Belarusian border. "The situation is not calm," Mariusz Blaszczak told Polish radio.
His ministry has accused Belarusian forces of firing shots into the air in an area where migrants have set up a makeshift camp. The ministry posted a video on Twitter with the sound of what sounds like a gunshot.
It is impossible to verify the information independently. Poland has imposed a state of emergency that prevents reporters, activists and any other non-residents from entering a border zone.
Lithuania declares state of emergency
In Lithuania, a state of emergency came into force at midnight, due to last for a month along its border with Belarus. At least 170 migrants were stopped from entering the country on Tuesday.
The measure restricts the movement of vehicles and imposes a ban on entry into a five-kilometre zone inland, residents excepted. Guards can carry out checks inside the area. It also bans gatherings in the zone and applies to migrants elsewhere too, including in the capital Vilnius.
The latest developments come amid the most tense period to date after months of heavy migration on Belarus' borders with Poland, Lithuania, and to a lesser extent, Latvia. The three European Union countries are located on the 27-nation bloc's eastern border.
Watch Euronews' interview with Linas Linkevičius, Lithuania's former Minister of Defence and former Minister of Foreign Affairs, in the video player above.
Merkel calls on Putin to intervene
The German chancellor’s office said Merkel spoke with Putin by phone and "underlined the fact that the instrumentalisation of migrants against the European Union by the Belarusian regime is inhuman and completely unacceptable, and asked the Russian president to exert his influence on the regime in Minsk".
Russia is a close ally of the government in Belarus. Germany is a favoured destination for migrants who arrive in the European Union.
The Kremlin's account of the call said Putin "proposed to establish a discussion of the problems that have arisen in direct contacts of representatives of the EU member states with Minsk", adding that Putin and Merkel “agreed to continue the conversation on the issue".
Minsk and Moscow reject accusations
Belarus continued its attempt to shift the blame onto Poland and the West on Wednesday, the day after Lukashenko accused Poland of conducting a "war" against the migrants.
Turning the accusations against Minsk on their head, Foreign Minister Vladimir Makeï said the migration crisis had been "provoked by the EU and its member states bordering Belarus", and was being used as an "excuse" for imposing a "fifth round of sanctions".
Speaking from Moscow as he met his Russian counterpart Sergei Lavrov, Makeï added that the two countries were working for a united response in the face of "unfriendly acts" targetting Belarus.
Lavrov concurred, stating that they had reinforced their collaboration "to counter a campaign against Belarus triggered by Washington and its European allies within international organisations".
The Kremlin has rejected accusations from Poland's prime minister, who claimed on Tuesday that President Putin was orchestrating the influx of migrants as the "sponsor" of Lukashenko's actions.
Dmitri Peskov described Mateusz Morawiecki's comments as "absolutely irresponsible and unacceptable".
Moscow's role in the influx of migrants has come under heightened scrutiny this week. The EU said on Tuesday that Russia is among 20 countries suspected of involvement in transporting people towards Belarus.
EU leaders allege Belarus is retaliating for sanctions the bloc imposed on the authoritarian government in Minsk, over its brutal crackdown on domestic dissent.
Thousands of people were jailed and beaten following months of protests in the wake of last year's presidential election that Lukashenko is widely believed to have rigged to win a sixth term.
EU funds for border barriers 'legally possible'
Appearing alongside Morawiecki in the Polish capital, Charles Michel raised the prospect of further EU sanctions against the regime in Belarus — as well as possible measures against the migrants' countries of origin and airlines transporting them to Europe.
The consequences of their decisions amounted to "a hybrid attack against the EU", said the President of the European Council, the body representing EU heads of state and government.
"If we are not able to convince with our arguments, we must be ready to decide concrete and operational sanctions because we need to be firm, what's happened is serious and we need to react firmly, we need to be united," he added.
Michel also raised the question of potential EU financing for physical border infrastructure in member states, which he said was "legally possible" according to the Council's legal opinion.
Lithuania, he pointed out, had a border 600-700 kilometres long with Belarus and was facing a "totally new situation".
"Is it possible for the EU to show solidarity by helping them to protect their national borders which are also European borders?" he asked, adding that he hoped the EU would be able to agree a clear line.
Twelve EU countries wrote to the Commission in October asking for EU financing for border infrastructure. Its President, Ursula von der Leyen, dismissed the idea, saying the EU would not fund "barbed wire or walls".