WASHINGTON: Senior U.S. officials on Sunday said they could not confirm reports that U.S. intelligence indicates that Russia is planning to invade Ukraine on Wednesday, but said they would try to prevent any "surprise attack" by sharing what they knew of Russia's plans.

White House National Security Adviser Jake Sullivan repeated that a Russian invasion could begin any day and President Joe Biden has said he will support Ukraine after any invasion and defend NATO territory.

"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," Sullivan told CNN's "State of the Union" when asked about the possible Wednesday timing.

"We will defend every inch of NATO territory, every inch of Article Five territory and Russia we think fully understands that message," Sullivan said in a separate interview with CBS' "Face the Nation" program.

Ukraine is not part of the NATO alliance.

Pentagon spokesperson John Kirby also on Sunday declined to confirm reports on the Wednesday timing.

"I'm not in a position to confirm those reports," Kirby said during an interview on "Fox News Sunday".

Kirby also said a Russian military action could take place any day.

"And again, these assessments are coming from a variety of sources. And not, not exclusively just inside intelligence, but also what we're seeing in plain sight," Kirby said. "More than 100,000 troops now continue to be arrayed against Ukraine's border."

Their comments came amid a flurry of diplomatic activity aimed at trying to resolve the West's standoff with Moscow over Ukraine to avoid military action.

U.S. President Joe Biden, who spoke with Russian President Vladimir Putin on Saturday, was due to speak with Ukraine's President Volodymyr Zelenskiy, U.S. and Ukrainian officials said.

Both Sullivan and Kirby repeated warnings for Americans to leave Ukraine.

"What we've seen just in the last 10 days or so is an acceleration of that buildup and movement of Russian forces of all varieties, closer to the border with Ukraine, in a position where they could launch a military action very, very rapidly," Sullivan said.

He added that the United States and its allies "will defend NATO territory, we will impose costs on Russia," in the event of a Russian attack.