US 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 Secretary of State Antony Blinken has also spoken to counterparts from G7 nations.
UK: Russian state was facing its greatest security challenge
NATO is "monitoring the situation" in Russia, a spokeswoman for the transatlantic military alliance said via email on Saturday. The spokeswoman, Oana Lungescu, had no further comment.
Britain's defense ministry said on Saturday that the Russian state was facing its greatest security challenge of recent times, following what it said appeared to be a move by Wagner Group mercenary forces towards Moscow.
Russian President Vladimir Putin said in an emergency televised address on Saturday that an "armed mutiny" by the Wagner Group was treason, and that anyone who had taken up arms against the Russian military would be punished.
"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how this crisis plays out," Britain's defense ministry said in a regular intelligence update.
"This represents the most significant challenge to the Russian state in recent times," it added.
Britain's foreign ministry warned of a risk of unrest across Russia, in an update of advice to travelers on Saturday following movements by Russia's Wagner Group of mercenary forces.
"There are reports of military tensions in the Rostov region and a risk of further unrest across the country. Additionally, there is a lack of available flight options to return to the UK," Britain's foreign ministry said.
Britain's government continued to advise against all travel to Russia.
British Prime Minister Rishi Sunak said on Saturday that he wanted all parties in Russia to protect civilians after the private army of mercenary boss Yevgeny Prigozhin seized control of a southern city.
"The most important thing I'd say is for all parties to be responsible and to protect civilians," he told the BBC in a television interview which will be broadcast in full on Sunday.
"We're keeping a close eye on the situation and how it's evolving on the ground as we speak. We're in touch with our allies and in fact, I will be speaking with some of them later today," he added.
The British government's emergency committee, known as Cobra, is meeting to discuss the ongoing situation in Russia, BBC reporter Laura Kuenssberg said on Saturday.
Kuenssberg interviewed British Prime Minister Rishi Sunak earlier on Saturday when he said he would speak with allies and urged all parties in the conflict in Russia to avoid harm to civilians.
G7 discuss the situation in Russia
US Secretary of State Antony Blinken said on Saturday he spoke with G7 foreign ministers and the EU high representative for foreign affairs after Wagner fighters took control of some military facilities in southern Russia.
"The United States will stay in close coordination with Allies and partners as the situation continues to develop," Blinken wrote on Twitter.
German Foreign Minister Annalena Baerbock has spoken to her counterparts within the Group of Seven industrialised nations about developments in Russia, the foreign ministry in Berlin said on Saturday.
"Foreign Minister Baerbock has just discussed the situation with the foreign ministers of the G7," said a ministry spokesperson, adding that the German government's crisis team was also meeting.