By Jasper Ward

WASHINGTON U.S. President Joe Biden spoke with the leaders of France, Germany and the United Kingdom on Saturday as mutinous Russian mercenaries barrelled towards Moscow after seizing a southern city overnight, the White House said.

"The leaders discussed the situation in Russia. They also affirmed their unwavering support for Ukraine," a readout said.

Biden was briefed about the unfolding situation in Russia by his national security team on Saturday morning, the White House said, adding that the president will continue to be briefed throughout the day.

A spokesperson for German Chancellor Olaf Scholz said that during the conversation, the four leaders had reaffirmed their commitment to continue supporting Ukraine for as long as necessary.

British Prime Minister Rishi Sunak's office made a similar commitment about Ukraine, and said the leaders "have agreed to stay in close contact in the coming days".

U.S. Secretary of State Antony Blinken has also spoken to counterparts from G7 nations.

The move by Wagner Group mercenaries presents Russian President Vladimir Putin with the first serious challenge to his grip on power in his 23-year rule.

(Reporting by Jasper Ward; Additional reporting by Andreas Rinke and David Milliken; Editing by Daniel Wallis and Frances Kerry)