Senior Russian security officials say the war in Ukraine is "lost", suggesting that Vladimir Putin's regime may come to an end, according to an expert on Russia-related security threats.

"The" informed elite "within the security forces" understands that the war is lost, "Bellingcat's chief investigator for Russia, Christo Grozev, was quoted as saying in an interview with Radio Liberty and Metro.

To have a chance to win the war, Grozev said, the Russian president would need full mobilization, but that would cause problems for him domestically.

Mass mobilization would lead to a "social explosion" in Russia, Grozev added, according to the Metro article, which was also referred to by Business Insider.

In this regard, there are those in Putin's inner circle who can pressure him to use nuclear or chemical weapons, Grozev continued, but others will say "enough is enough."

"These people would say, 'It's better not to lose another 10,000 lives of our soldiers and officers,'" Grozev was quoted as saying.

Although exact figures are not available, it is estimated that thousands of Russian servicemen have died in the country's brutal offensive in Ukraine.

In fact, according to the UK Ministry of Defense, Russia has lost a third of its forces.

Western officials say Russia, facing military setbacks, is losing momentum as the war in Ukraine continues.

And Grozev has said that security officials in the FSB, who know how many Russian soldiers have died, think Putin is losing control of his power.

"These are the parts of the security forces that know the dangers for the regime and they themselves are now preparing for their future," he told Metro.

Otherwise, days ago, Ukraine's military intelligence chief told Sky News that a coup to overthrow Putin was already under way.

"It is impossible to stop him," said Major General Kyrylo Budanov.