Lloyd Austin, the US defense secretary, said during a news conference on Saturday that Russia's military was prepared to strike Ukraine.
Austin told reporters that Russian troops are moving closer to the Ukrainian border.
According to Irish national broadcaster RTE, he said that troops were moving into the right kinds of positions to be able to conduct an attack.
A US defense official told Sky News that up to 50% of the Russian troops are in attack positions. According to the British media outlet, an estimated 150,000 troops are in the area.
President Joe Biden said on Friday that he is convinced that a Russian invasion into Ukraine will take place in the coming days. He said that the capital of Ukraine would be targeted.
Sky News defense editor Deborah Haynes said that at Saturday's news conference that an invasion is not inevitable. He noted that Putin could still choose a different path.
Austin told reporters that he hopes he steps back from the brink of conflict.
Western leaders will meet in Germany on Saturday to discuss how to protect Ukraine from a Russian invasion.
Multiple explosions were heard in the rebel-held Ukrainian city of Donetsk on Saturday. It is not known what caused the blasts. One of its soldiers was killed.
Biden has accused Russian-backed rebels in the region of being part of a plan to create a text for an invasion of Ukraine.
The Kremlin is not planning to invade.