US National Security Adviser Jake Sullivan on Sunday warned that the United States believes Russia could launch an invasion of Ukraine this week, but is still holding out hope diplomacy can prevail.

In an interview with CNN, Sullivan said that Russian forces are positioned so that an invasion could take place before the end of the Beijing Winter Olympics, which end on February 20.

"We cannot perfectly predict the day, but we have now been saying for some time that we are in the window, and an invasion could begin, a major military action could begin by Russia in Ukraine any day now -- that includes this coming week, before the end of the Olympics," he said.

"The way they have built up their forces, the way they have maneuvered things in place, makes it a distinct possibility there will be major military action very soon. And we are prepared to continue to work on diplomacy, but we are also prepared to respond in a united and decisive way with our allies and partners should Russia proceed," added Sullivan.

Sullivan was speaking a day after President Joe Biden warned Russian President Vladimir Putin of "swift and severe costs" if he were to order an invasion of Ukraine.

Meanwhile, Pentagon press secretary John Kirby said Sunday the intelligence the Defense Department has that suggests Russia could invade Ukraine at any moment "really was a combination of factors," including what the US is seeing in plain sight on the border.

"I think a mosaic of the intelligence that we're seeing, not speaking to it specifically, but we have good sources of intelligence and they're telling us that things are sort of building now to some kind of crescendo opportunity for Mr. Putin," he told Fox.

Tensions between Russia and Ukraine have mounted in recent weeks, after Russia amassed tens of thousands of troops on its border with Ukraine.

On Friday, US Secretary of State Antony Blinken warned that Russia could launch an invasion of Ukraine at any time, including during the Beijing Winter Olympics.

Blinken said Washington was continuing to "draw down" its embassy in Ukraine and repeated a State Department call to American citizens in Ukraine to leave the country immediately.

"Simply put, we continue to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border," he said.

On Thursday, US President Joe Biden issued a warning to any Americans who remain in Ukraine as Russia continues to threaten an invasion: Leave.

"American citizens should leave now," Biden said in an interview with NBC News, adding, “It’s not like we’re dealing with a terrorist organization. We’re dealing with one of the largest armies in the world. It’s a very different situation and things could go crazy quickly.”

Asked what scenario could prompt him to send troops to rescue Americans fleeing the country, Biden replied, “There’s not. That's a world war when Americans and Russia start shooting at one another.”

The interview came as the State Department issued an advisory stating that the US “will not be able to evacuate US citizens in the event of Russian military action anywhere in Ukraine.”

It warned that regular consulate service — including aiding citizens trying to leave the country — would be “severely impacted.”