US Secretary of Defence Lloyd Austin also says a conflict is "not inevitable", adding that Russian President Vladimir Putin "can choose a different path."
US Secretary of Defence Lloyd Austin has said Russian troops on Ukraine's border are "uncoiling" and "poised to strike", stepping up warnings of an invasion by Moscow.
The troops are "moving into the right kinds of positions to be able to conduct an attack", Austin said on Saturday during a visit to Lithuania.
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."
Austin said he agreed with US President Joe Biden that Putin had "made the decision" to invade.
US Vice President Kamala Harris warned Moscow that any invasion will invite not only damaging economic sanctions but also a bolstered NATO on Europe's eastern flank.
"We will not stop with economic measures. We will further reinforce our NATO allies on the eastern flank" in response to an invasion, Harris said at the annual Munich Security Conference.
The dramatic US warnings, increased shelling on the front lines and evacuation of civilians from Russia-backed rebel regions in Ukraine have come together to raise the fear of a major conflict.
Invasion fears grow
Exacerbating the febrile situation, Russia was staging another show of military might on Saturday with President Putin overseeing drills involving nuclear-capable missiles.
Russia's defence ministry said the "planned exercises" would involve nearly all branches of Russia's armed forces.
Ukrainian President Volodymyr Zelenskyy meanwhile was headed to Germany to shore up support among Western allies, despite a significant increase in clashes in the country's east.
The volatile front line between Ukraine's army and separatists in the Moscow-backed breakaway regions of Donetsk and Luhansk has seen a "dramatic increase" in ceasefire violations.
Hundreds of artillery and mortar attacks were reported in recent days, in a conflict that has already rumbled on for eight years and claimed the lives of more than 14,000 people.
The Kremlin insists it has no plans to attack Ukraine, which has angered Moscow by seeking long-term integration with NATO and the European Union.
Moscow is demanding written guarantees that its neighbour will never be allowed to join NATO, and for the US-led military alliance to roll back deployments in eastern Europe.