The warning comes as the region braces for further confrontation after Vladimir Putin received authorisation to use military force outside his country and the West responded with sanctions.
A senior US defence official in Washington said the Russian forces arrayed along Ukraine’s borders are “as ready as they can be” for an invasion, with about 80% in what the US considers "forward positions, ready to go” within 5 kilometres to 50 kilometres of the border.
"We still cannot confirm that Russian forces have moved into the Donbas area,” added the official, who spoke on condition of anonymity to discuss sensitive information.
The warning comes as the region braced for further confrontation after Russian President Vladimir Putin received authorisation to use military force outside his country and the West responded with sanctions. Ukraine urged its citizens to leave Russia on Wednesday and is moving towards a state of emergency.
Donetsk rebel leader Denis Pushilin insisted Wednesday there were no Russian troops in the region even though a local council member claimed the previous day they had moved in.
Putin said on Tuesday that he hadn’t yet sent any Russian troops into the rebel regions, contrary to Western claims. US President Joe Biden said he estimated that the invasion had already "begun", though Washington said it did not yet have independent proof of Russian troops inside Ukrainian territory.
In Ukraine's east, where an eight-year conflict between Russia-backed separatists and Ukrainian forces has killed nearly 14,000 people, violence also spiked again. One Ukrainian soldier was killed and six more were injured after rebel shelling, the Ukrainian military said. Separatist officials reported several explosions on their territory overnight and three civilian deaths.
Russia emptied its diplomatic posts in Ukraine, state news agency Tass reported, a day after the Foreign Ministry announced a plan to evacuate, citing threats. By Wednesday afternoon, the Russian flag no longer flew over the Kyiv embassy, and police surrounded the building.
Earlier, Vladimir Putin marked a national day of celebration. The Russian leader attended a wreath-laying ceremony on the annual Defender of the Fatherland Day.
But, as the Kremlin was hit with a fresh wave of sanctions, Putin reaffirmed that Russia's interests remained non-negotiable.
During a call with Turkey's president, he doubled down on his rhetoric accusing Ukraine of breaking the Minsk agreement. Nevertheless, despite increasing uncertainty, the Kremlin insists it is still prepared for frank discussions with the West.
"Our country is always open to a direct and honest dialogue and ready to search for diplomatic solutions to the most complicated issues. But I want to repeat that Russia's interests and the security of our people are an indisputable priority," Vladimir Putin said.
In St Petersburg, meanwhile, several hundred people reportedly rallied in support of the self-proclaimed Donetsk and Luhansk republics in eastern Ukraine.
After weeks of rising tensions, Putin took a series of steps this week that dramatically raised the stakes. First, he recognised the independence of those separatist regions. Then, he said that recognition extends even to the large parts of the territories now held by Ukrainian forces, including the major Azov Sea port of Mariupol.
Finally, lawmakers gave him authority to use military force outside the country — effectively formalising a Russian military deployment to the rebel regions.
Putin laid out three conditions that he said could end the standoff, urging Kyiv to recognise Russia’s sovereignty over Crimea, the Black Sea peninsula that Moscow annexed from Ukraine in 2014, to renounce its bid to join NATO and partially demilitarise. Ukraine long has rejected such demands.