The White House on Friday warned a Russian invasion of Ukraine could involve a military assault on the Ukrainian capital of Kyiv.

"We’ve been clear that it could take a range of different forms, but I want to be equally clear that one of those forms is a rapid assault on the city of Kyiv," national security adviser Jake Sullivan told reporters at a White House briefing.

"That is a possible line of attack, course of action, that the Russian forces could choose to take. They could also choose to move in other parts of Ukraine as well," Sullivan said.

An attack on Kyiv could be devastating for Ukraine. It is the seat of the Ukrainian government and the most populous city in the country at nearly 3 million people.

Sullivan said officials have not determined that Russian President Vladimir Putin has decided to invade Ukraine, but intelligence indicates Moscow has accumulated enough forces at the Ukrainian border that an invasion could happen before the Winter Olympics conclude on Feb. 20.

"It could be more limited, it could be more expansive, but there are very real possibilities that it will involve the seizure of a significant amount of territory in Ukraine and the seizure of major cities, including the capital city," Sullivan said when asked about the potential course of a military invasion.

Biden administration officials have been sounding the alarm for weeks that a Russian invasion of Ukraine was possible as Moscow amassed troops and military equipment at the border of the two countries. They have pledged to levy severe economic sanctions against Russia should it act, and officials have projected unity with NATO allies.

Friday's comments from Sullivan, which echoed those of Secretary of State Antony Blinken, marked a new level of urgency about the imminent threat of a Russian attack. President Biden and other officials have urged Americans in Ukraine to leave the country immediately, citing the risk of violence and difficulty getting out should fighting start.