U.S. national security adviser Jake Sullivan gives a statement about the situation in Afghanistan during a news briefing at the White House in Washington, U.S., August 23, 2021.
U.S. national security adviser Jake Sullivan gives a statement about the situation in Afghanistan during a news briefing at the White House in Washington, U.S., August 23, 2021. Reuters / LEAH MILLIS

Russia now has enough forces to conduct a major military operation against Ukraine and an assault could begin "any day now," White House national security adviser Jake Sullivan said on Friday.

Sullivan, speaking at a White House briefing without listing specific evidence, said any American still in Ukraine should leave in the next 24-48 hours. He said a Russian invasion could start with an air assault that would make departures difficult.

Sullivan said U.S. intelligence believes Russian President Vladimir Putin could order an invasion before the end of the Winter Games in Beijing and that a rapid assault on the Ukraine capital Kyiv is a possibility.

Sullivan spoke after President Joe Biden held a secure video call with transatlantic leaders from the White House Situation Room and sought allied unity in the face of a worsening situation.

It remains unclear, Sullivan said, whether Putin has definitively given an order to start an invasion. He said he expects Biden to seek out a phone call with Putin soon on the crisis.