Russia could invade Ukraine 'at any time' now, warns US

Russia could invade Ukraine "at any time" and American citizens should leave immediately, the US has warned.

An invasion could start with aerial bombing that would make departures difficult and endanger civilians, the BBC quoted the White House as saying on Friday.

Moscow has repeatedly denied any plans to invade Ukraine despite massing more than 100,000 troops near the border.

The US statement prompted countries around the world to issue fresh warnings to nationals in Ukraine, the report said.

The UK, Australia, Canada and the Netherlands are among those urging citizens to leave as soon as possible.

However, attempts to de-escalate tensions through diplomacy are set to continue on Saturday, with both US President Joe Biden and France's Emmanuel Macron due to speak to Russia's Vladimir Putin by phone.

Moscow, meanwhile, has accused Western countries of stirring up hysteria.

US National Security Adviser Jake Sullivan said Russian forces were now "in a position to be able to mount a major military action" in remarks seen as a clear escalation in the urgency of warnings from US officials, the BBC reported.

"We obviously cannot predict the future, we don't know exactly what is going to happen, but the risk is now high enough and the threat is now immediate enough that (leaving) is prudent," he said.

Sullivan added that the administration did not know if Putin had made a final decision to invade, but said that the Kremlin was looking for a pretext to justify military action, which he said could start with intense aerial bombardment.