US Secretary of State Antony Blinken on Friday said Russia is amassing yet more troops on Ukraine's border, and warned an invasion could come "any time", including during the Winter Olympics which end on February 20.
"We're in a window when an invasion could begin at any time, and to be clear that includes during the Olympics," he said, brushing aside suggestions that Moscow would wait until after the Beijing Games to avoid upstaging its ally China.
"Simply put, we continue to see very troubling signs of Russian escalation," Blinken said after a meeting of counterparts from the so-called Quad countries -- Australia, India, Japan and the United States -- in Melbourne.
Blinken insisted that the United States "would strongly prefer to resolve the differences" with Russia "through diplomacy".
"We've made every possible effort to engage Russia," he said.
"But at the same time, we've been very clear in building deterrence and building defence and making it clear to Russia that if it chooses the path of renewed aggression, it will face massive consequences."
His comments come as President Joe Biden urged Americans in Ukraine to immediately leave the country, another strong public signal that war may be drawing closer.
"American citizens should leave, should leave now," Biden said in an interview with NBC News.
"We're dealing with one of the largest armies in the world. This is a very different situation and things could go crazy quickly."