US Secretary of State Antony Blinken has said Russia could attack Ukraine "on very short notice" and warned again of tough sanctions if it did.
He was speaking on a visit to the Ukrainian capital Kyiv after months of tension over a Russian military build-up near Ukraine's borders.
Stressing the US's close ties with Ukraine, Mr Blinken vowed "relentless" diplomacy to stop Russian aggression.
Moscow has denied any plans to attack or invade.
Russia has made a raft of demands to Western governments, including that Ukraine should never join Nato and that the defensive alliance's military activities should be limited in member states including Poland.
Talks between the West and Russia last week failed to reach a breakthrough, with some of Moscow's demands rejected as non-starters.
Russia annexed Ukraine's Crimean peninsula in 2014 after the overthrow of a pro-Russian government in Ukraine. It has supported pro-Russian rebels who control parts of eastern Ukraine after they fought a bloody war with government forces.
There are fears that the conflict, which cost at least 13,000 lives and caused at least two million people to flee their homes, may reignite, with the Russian military openly intervening.