One of Moscow's most senior military commanders was arrested on Thursday after Vladimir Putin promised to "purify" Russia of traitors in a sign of "real discord" in the Kremlin over the war, according to a British minister.
It came as US intelligence said at least 7,000 Russian troops have been killed in Ukraine, including three generals, with up to 21,000 soldiers injured.
That "conservative" estimate is greater than the number of American troops killed over 20 years in Iraq and Afghanistan combined.
The losses appear to be leading to a purge of Putin's top military and intelligence commanders, including General Roman Gavrilov, the deputy head of Russia's National Guard, on Thursday.
Increasingly isolated and angry, the Russian president this week used a TV appearance to lash out at "traitors".
"Russian people are able to distinguish between true patriots and scum and traitors, and simply spit them out like a fly that flew into their mouths," Putin told an online meeting of the Russian cabinet.
"I am convinced that a natural and necessary self-purification of society will strengthen our country."
James Heappey, the UK Armed Forces minister, said the language was "fanatical" and "hugely dangerous".
"There's a desperation that might make him consider a course of action we would view as very dangerous indeed," Mr Heappey told The Telegraph. "I'm deeply concerned about where his state of mind is at and how desperate he may become."
He added that the developments within the Kremlin were evidence of "real discord" at the top of the Russian high command.
Gen Gavrilov was reportedly arrested either because of "leaks of military info that led to loss of life" or the "wasteful squandering of fuel", according to Christo Grozev, the chief Russia investigator at the Bellingcat open-source intelligence reporting project.
"One thing is clear: it's doubtless that Putin recognises the deep s--- this operation is in," he wrote on Twitter. "It's so bad that he changes horses in mid stream – a big no-no during war."
Gen Gavrilov had previously worked as a commander in the FSO, the military unit tasked with guarding the president. Russia's National Guard has been fighting in Ukraine and has reportedly suffered heavy casualties.
Kremlin paranoia has led to purges of top officials throughout history. Soviet dictator Joseph Stalin, paranoid about rival leadership groups, was infamous for sending senior officials to gulags in Siberia. He also blamed and purged the military high command in 1941 after Nazi Germany surprised him with its mass invasion of the Soviet Union.
Talk from the Kremlin of cleansing Russia continued on Thursday with Dmitry Peskov, Putin's spokesman, describing Russians fleeing abroad as weak. "Many are showing their essence. They are traitors," he said. "That's how Russia is cleansed."
Over the weekend, Putin also arrested the head and deputy head of a unit within the FSB, Russia's spy organisation, which had been responsible for pre-invasion intelligence in Ukraine.
Andrei Soldatov and Irina Borogan, two Russian journalists considered best connected to Russia's intelligence community, said Colonel-General Sergei Beseda, the head of the FSB's Fifth Service, and his deputy, Anatoly Bolyukh, had been arrested for corruption and intelligence failings.
"It looks like two weeks into the war, it finally dawned on Putin that he was completely misled. The department, fearful of his responses, seems to have told Putin what he wanted to hear," they wrote in an article for the Centre for European Policy Analysis, a Washington-based think tank.
Muddled tactics, poor logistics and an over-reliance on unmotivated conscripts have combined to slow Russia's military in Ukraine.
Analysts have said Putin's increasingly erratic outbursts on television suggest that he is becoming increasingly paranoid and isolated.
In his rant against traitors he also derided the Russians who he said preferred the delicacies of the decadent West over supporting Russia in her hour of need.
"I do not at all judge those who have a villa in Miami or on the French Riviera, who can't do without foie gras, oysters or gender freedoms," he said. "But the issue here is that many of these people are by their very nature, mentally located there and not here with our people, not in Russia."
