Russia could invade Ukraine within days or weeks but could still opt for a diplomatic path, White House national security adviser Jake Sullivan said on Sunday.

"We are in the window. Any day now, Russia could take military action against Ukraine, or it could be a couple of weeks from now, or Russia could choose to take the diplomatic path instead," Sullivan told the "Fox News Sunday" program.

Sullivan made the comments in television interviews after two U.S. officials on Saturday said Russia has in place about 70 percent of the combat power it believes it would need for a full-scale invasion of Ukraine.

Any possible Russian action could include annexing Ukraine's Donbass region, where Russian-backed separatists broke away from Ukrainian government control in 2014, cyberattacks or a full-scale invasion of Ukraine, Sullivan added, saying Russia could act as soon as on Monday though it could also be weeks.

"We believe that there is a very distinct possibility that Vladimir Putin will order an attack on Ukraine," he told ABC's "This Week" program.

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"It could take a number of different forms. It could happen as soon as tomorrow, or it could take some weeks yet. He has put himself in a position with military deployments to be able to act aggressively against Ukraine at any time now," Sullivan added.

As Russia masses more than 100,000 troops near the border, Moscow has said it is not planning an invasion but could take unspecified military action if its security demands are not met.

Those include a promise that NATO will never admit Ukraine, a demand the United States and NATO have called unacceptable.

Asked if a Russian invasion was likely, U.S. Ambassador to the United Nations Linda Thomas-Greenfield told CNN in an interview: "We're still working to discourage the Russians from making the wrong choice of choosing confrontation."

Ukrainian Foreign Minister Dmytro Kuleba on Sunday urged people to ignore "apocalyptic predictions" about an imminent Russian invasion, saying his country was strong and had unprecedented international support.

President Volodymyr Zelenskiy said a week ago Ukraine was not a sinking Titanic and accused Washington and media of fuelling panic that weighed on the economy when there were "no tanks in the streets".

"Today, Ukraine has a strong army, unprecedented international support and Ukrainians' faith in their country," Kuleba said in a tweet.

"The enemy should be afraid of us, not us of them."