"We do have information that the Russians are likely to want to fabricate a pretext for an invasion," said Pentagon spokesperson John Kirby.

He told reporters that Washington believed the Russian government plans to stage an attack by the Ukraine military or intelligence forces "against Russian sovereign territory, or against Russian speaking people."

The latter could refer to the sizeable Russian-speaking population inside Ukraine.

"As part of this fake attack, we believe that Russia would produce a very graphic propaganda video, which would include corpses and actors that would be depicting mourners and images of destroyed locations," he said.

That could allow Moscow, which has amassed more than 100,000 troops and heavy offensive arms on Ukraine's border, with an excuse for invading.

- Question of evidence -

Neither Kirby nor State Department Spokesperson Ned Price, who also commented on the alleged plan, offered evidence to back the claim.

Kirby said part of the plan would be to make the Ukrainian military equipment used in it appear to be supplied by the West, he said, further justifying Russian reprisals against Ukraine.

"We've seen these kinds of activity by the Russians in the past and we believe it's important when we see it like this that we can call it out," Kirby said.

"I would just say that our experience is that very little of this nature is not approved at the highest levels of the Russian government," Kirby said about the purported plan.

Price said the alleged plan is "one of a number of options that the Russian government is developing as a fake pretext to initiate and potentially justify military aggression against Ukraine."

He said the United States did not know if Moscow has decided to go through with the plan.

"Russia has signaled it's willing to continue diplomatic talks as a means to de-escalate, but actions such as these suggests otherwise," Price said.

Pressed on whether there was evidence of such a plan, Price said it came from US intelligence, but offered no more details.

"I'm not going to spell out what is in our possession but I will leave that to your judgement," he told reporters.

British Foreign Secretary Liz Truss called the US claim of Moscow's false flag operations "clear and shocking evidence of Russia's unprovoked aggression and underhand activity to destabilize Ukraine."

"The only way forward is for Russia to de-escalate, desist and commit to a diplomatic pathway," she said in a tweeted statement.