Prime Minister Rishi Sunak said he is in touch with allies about the armed rebellion led by the Wagner mercenary group in Russia, which UK defence officials have described as "the most significant challenge" to the Kremlin in recent times.
President Vladimir Putin called the actions of the private military company a "betrayal" and vowed to defend Russia, after mercenary chief, Yevgeny Prigozhin, vowed to topple Moscow's military leadership.
Foreign Secretary James Cleverly chaired a meeting of the Government's emergency Cobra committee to discuss the fast-moving situation.
The Prime Minister urged all parties involved to protect civilian lives, as he indicated he will speak to Ukrainian president Volodymyr Zelenksy and other western leaders on Saturday.
"We're keeping a close eye on the situation, as it's evolving on the ground as we speak," Mr Sunak told the BBC.
"The most important thing I'd say is for all parties to be responsible and to protect civilians, and that's about as much as I can say at this moment."
Asked whether he had spoken to Mr Zelenksy, Mr Sunak said: "I'm in touch with our allies.
"I'll be speaking to some of them later today, as you would expect us to be co-ordinated on a situation like this, but it is evolving as we speak."
He declined to say whether it is good or bad news that Mr Putin is being challenged.
Pressed on advice for British nationals remaining in Russia, Mr Sunak said the UK has "had long-standing travel advice against travel to Russia" and "people should keep checking the Foreign Office website for updates".
Mr Cleverly tweeted that "we are monitoring the situation carefully and liaising closely with our allies" and "we continue to urge British citizens to follow FCDO (Foreign, Commonwealth and Development Office) travel advice".
An FCDO spokesperson said: "The Foreign Secretary has chaired a meeting of COBR to update on the latest situation, particularly with respect to British nationals in Russia."
Earlier, the Ministry of Defence (MoD) said that "over the coming hours, the loyalty of Russia's security forces... will be key to how the crisis plays out" as Wagner forces appear to be moving towards the Russian capital.
In its latest intelligence briefing, the department said the feud between the Wagner group and the Russian defence ministry "escalated into outright military confrontation" in the early hours of Saturday.
"In an operation characterised by Prigozhin as a 'march for freedom', Wagner Group forces crossed from occupied Ukraine into Russia in at least two locations," the MoD said.
It said that in the southern Russian city of Rostov-on-Don, "Wagner has almost certainly occupied key security sites", including the Russian military headquarters that oversees the fighting in Ukraine.
"Further Wagner units are moving north through Vorenezh Oblast, almost certainly aiming to get to Moscow. With very limited evidence of fighting between Wagner and Russian security forces, some have likely remained passive, acquiescing to Wagner.
"Over the coming hours, the loyalty of Russia's security forces, and especially the Russian National Guard, will be key to how the crisis plays out.
"This represents the most significant challenge to the Russian state in recent times."
Wagner troops have been fighting alongside Russian soldiers in Ukraine and succeeded in taking the eastern city of Bakhmut, where the bloodiest and longest battles have raged.
But Mr Prigozhin has stepped up his criticism of Russia's military leadership, accusing it of botching the war and shelling his fighters.
The escalation of Russian infighting could further hamper Moscow's war effort as the early stages of a Ukrainian counter-offensive unfold.
Amid the rapidly-evolving events in Russia, Mr Zelensky said Moscow is suffering "full-scale weakness".
Conservative chairman of the Commons Defence Committee Tobias Ellwood described the developments as a "huge opportunity for Ukraine to exploit the current mutiny and chaos in Russia".
