Russian troops on Ukraine's border are "uncoiling" and "poised to strike", US Secretary of Defense Lloyd Austin said on Saturday during a visit to Lithuania.
"They are uncoiling and are now poised to strike," he said, adding that troops were "moving into the right kinds of positions to be able to conduct an attack".
But he said conflict was "not inevitable", adding that Russian President Vladimir Putin "can choose a different path".
"The US, in lockstep with our allies and partners, have offered him an opportunity to pursue a diplomatic solution. We hope that he takes it. We hope that he steps back from the brink of conflict."
Austin said he agreed with US President Joe Biden that Putin had "made the decision" to invade.
"There are significant combat forces forward, those forces are now beginning to uncoil and move closer to the border, that facilitates their onward movement," he said.
"Having done this before, I can tell you that is exactly what you need to attack and the stance that you need to be in to attack," said Austin, who served as US commander in Iraq.
Austin also assured the Baltic states of Estonia, Latvia and Lithuania that the US "stands with our allies" and its commitment to NATO's collective defence was "iron-clad".
The current security tensions in the region have rattled the formerly Soviet-ruled Baltic states, which all have borders with Russia.
Alongside Austin, Lithuanian Foreign Minister Gabrielius Landsbergis said the Russian troop build-up around Ukraine was "a direct military threat to Ukraine but it also poses a threat to the entire region".
"The battle for Ukraine is a battle for Europe. If he is not stopped there, he will go further," Landsbergis said, referring to Putin.
The Baltics have asked NATO allies to send reinforcements to their countries on top of the existing alliance battle groups deployed there after Russia's annexation of Crimea in 2014.
Britain and Germany have pledged extra troops for the region, while the US has sent 4,700 reinforcements to neighbouring Poland, which also shares a border with Belarus and Russia.
The United States and Ukraine say there are now around 150,000 Russian troops massed on Ukraine's borders in Russia and Belarus.
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