President Joe Biden said there is a "very high" risk of a Russian invasion of Ukraine in the next few days.
"My sense is this will happen in the next several days," Biden told reporters outside the White House on Thursday.
The president also said he has "no plans" to speak to Russian leader Vladimir Putin, but said there is still a "clear diplomatic path" to avoid conflict.
US Ambassador to the United Nations Linda Thomas-Greenfield called this week a "crucial moment" in the tensions as Biden said he sent Secretary of State Antony Blinken to the UN on Thursday.
"The evidence on the ground is that Russia is moving toward an imminent invasion," Thomas-Greenfield wrote on Twitter.
Moscow claimed on Tuesday that it was reducing its troop presence along the border with Ukraine, but both the US and NATO have shown increasing skepticism.
This story is developing. Please check back for updates.