WASHINGTON (AFP) - The United States has not seen evidence that troops from close Russian ally Belarus are in Ukraine, the Pentagon said on Friday (March 11).

"We haven't seen any indications that Belarusian troops or forces have moved inside Ukraine," Pentagon spokesman John Kirby told reporters.

"We're not tracking any imminent involvement by Belarusian forces," Kirby said.

"That's not to say that it couldn't happen or that it wouldn't happen."

The Pentagon spokesman said the United States had noted comments by Belarusian President Alexander Lukashenko that his country's forces would "come to the defence of the rear of Russian forces should they be attacked."

"That's the first time that he's ever said anything about Belarus perhaps getting involved," Kirby said, but there are "no indications at this time that they have or that that's in the offing."

A senior US defence official said earlier on Friday that Russian forces have launched more than 80 missiles into Ukraine from Belarus.

That represents about 10 per cent of the more than 800 Russian missiles fired into Ukraine since the invasion began, the official said.

Russian troops have fired nearly 400 missiles from inside Ukraine while just under half have been fired from Russia, the official said.