Russian President Vladimir Putin could order an attack on Ukraine within days or weeks, White House national security adviser Jake Sullivan warned on Sunday, as Washington and its European allies continued efforts to offer Putin a diplomatic way out of the crisis.

“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 programme.

Sullivan made the comments in television interviews after two US officials on Saturday said Russia, which seized Crimea from Ukraine in 2014, has in place about 70 per cent of the combat power it believes it would need for a full-scale invasion of Ukraine.

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 US and the 30-nation western security alliance have called unacceptable.

US President Joe Biden spoke to French President Emmanuel Macron on Sunday, the White House said. The 40 minute-long call between Biden and Macron allowed the two leaders to coordinate ahead of the trip, a French Presidency source said.

If Putin is not deterred by the diplomatic push, 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 said.

“We believe that there is a very distinct possibility that Vladimir Putin will order an attack on Ukraine,” Sullivan told ABC’s This Week programme.

“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.