WASHINGTON: The top military officer in the United States canceled a trip to the Middle East on Saturday as the crisis in Russia intensified, a spokesman said.

Army General Mark Milley, chairman of the Joint Chiefs of Staff, had been due to travel to Israel and Jordan. That trip was postponed "due to the situation in Russia," a spokesman for the Joint Chiefs told AFP.

Jake Sullivan, US President Joe Biden's national security advisor, also canceled a trip Saturday, instead accompanying the president to Camp David.

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Sullivan had been scheduled to attend a conference on Ukraine in Denmark.

Milley had been due to leave for the Middle East on Saturday.

In Moscow, President Vladimir Putin is facing the biggest threat to his rule yet with rebel mercenaries advancing towards the Russian capital after seizing a key military base.

Earlier, the White House said Biden had discussed the unfolding crisis with the leaders of France, Germany and the United Kingdom, affirming their staunch support for Ukraine.

A White House spokesman also said Biden and Vice President Kamala Harris were briefed by their national security team Saturday morning on the Russian crisis and will continue to be briefed throughout the day.