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.

US to coordinate with allies amid Russia crisis: State Dept

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.

Putin, who described the Wagner Group’s actions as “treason” in an emergency televised address, said anyone who had taken up arms against the Russian military would be punished.