Shafaq News/ British Prime Minister Boris Johnson and U.S. President Joe Biden on Monday agreed there was a crucial window for diplomacy and a chance for Russia to step back and avert disaster in Ukraine, Johnson's office said after a call between the leaders.

"They agreed there remained a crucial window for diplomacy and for Russia to step back from its threats towards Ukraine," a Downing Street spokesperson said in a statement after the call.

"The leaders emphasised that any further incursion into Ukraine would result in a protracted crisis for Russia, with far reaching damage for both Russia and the world."

Pentagon spokesperson John Kirby  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."

The 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.

Reuters reported on Friday that the Pentagon will send 3,000 additional troops to Poland as Russia held military exercises in Belarus and the Black Sea following the buildup of its forces near Ukraine.

Moscow has repeatedly disputed Washington's version of events, saying it has massed more than 100,000 troops near the Ukrainian border to maintain its own security against aggression by NATO allies.