Ukraine’s top military expert has said that Kyiv has to make a “strategic, principal decision” on how to benefit from the unfolding turmoil in Russia.

One such move could be an order to invade western Russia to bypass massive defence installations on the 1,000-kilometre-long (620-mile-long) front line in eastern and southern Ukraine, according to Lieutenant General Ihor Romanenko.

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Moscow mayor Sergei Sobyanin has warned the situation in the capital was “difficult,” as forces of the Wagner mercenary group moved towards the city to oust Russia’s military leadership.

“The situation is difficult. I ask you to refrain from travelling around the city as much as possible,” Sobyanin said in a statement, warning of possible road closures and announcing Monday was a “non-working” day.

Kyiv’s military commander-in-chief Valery Zaluzhny has told the US chairman of the joint chiefs General Mark Milley that Ukraine’s counteroffensive against invading Russian troops “was going according to plan”.

“We discussed in detail the situation along the entire frontline,” Zaluzhny posted on official social media on Saturday. “I told him about the offensive and offensive actions of our units. I informed him that the operation is going in accordance with the plan.

“I also told him about the needs of the Armed Forces of Ukraine in certain types of weapons and demining assets.”

France advised against all travel to Russia given the “high volatility” of the military and security situation in the country, its foreign ministry has said on its website.

President Vladimir Putin has vowed to crush what he called an armed mutiny after rebellious mercenary chief Yevgeny Prigozhin said he had taken control of Rostov-on-Don, the capital of Russia’s southern region of Rostov.

Rostov-on-Don, a city which lies more than 1,000km (660 miles) south of Moscow, is the base for the military headquarters of Russian operations in Ukraine.

Rostov is Russia’s second-largest grain producing region and home to the Don River ports, which connect vessels exporting grain with the Azov Sea.

The British government’s emergency committee, known as COBR, has met to discuss the latest developments in Russia and the risks to British citizens there, a government 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,” a government spokesperson said.

British Foreign Secretary James Cleverly also took part in a meeting with counterparts from the Group of Seven major advanced economies.

The United Kingdom also repeated its advice that its citizens should not travel to any part of Russia and that those already there should leave.

“If your presence in Russia is not essential, we strongly advise that you consider leaving by remaining commercial routes,” the UK said in a travel advisory.

Wagner mercenaries have been promised an amnesty if they lay down their weapons but they need to act fast, Russia’s state-owned TASS news agency has cited a lawmaker as saying.

“Wagner fighters can still lay down their arms and avoid punishment given their achievements during the special military operation [in Ukraine], but they should do it fast,” TASS quoted the lawmaker, Pavel Krasheninnikov, as saying on Saturday.