Cherkesov was a mentor of Putin and his ‘unexplained death’ was announced via Telegram (Picture: east2west news)

A close KGB spy mentor of Vladimir Putin’s who suffered demotion after publicly criticising the Kremlin leader has died from an unexplained ‘serious disease’ .

Ruthless Viktor Cherkesov, 72, once in charge of crushing Soviet dissidents, was crucial to the Russian president’s rise but the pair later had strained relations.

Aleksandr Khinshtein announced the passing of Cherkesov on Telegram earlier today, but did not mention the cause of death.

Cherkesov served as a KGB officer in Putin’s native St. Petersburg during the Soviet era and later led the Federal Security Service (FSB) directorate in that city.

When Putin headed the FSB secret service before becoming prime minister, Cherkesov was installed as his trusted deputy.

No further details were immediately given on Cherkesov’s cause of death in St Petersburg.

In 2007, Cherkesov had suggested in a newspaper that the secret services were becoming corrupted under Putin.

Cherkesov (L) was Putin’s (R) deputy when he was head of the FSB (Picture: east2west news)

Cherkesov publicly criticised Putin and his leadership of the Kremlin (Picture: east2west news)

Cherkesov was fired from this role in the wake of his criticism of the secret services under Putin (Picture: east2west news)

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Instead of being ‘warriors’, senior officers were enriching themselves as ‘merchants’, he warned.

An outraged Putin blamed him for washing dirty linen in public, and hinted his longtime ally Cherkesov was hardly clean.

‘I consider it incorrect to air such problems in the media,’ he said.

‘And if someone acts in this way, makes such claims about a war of the special services, he himself must first be impeccable.’

At the time an agent and also a former agent working for Cherkesov – then Director of the Federal Drug Control Service – died mysteriously from ‘poisoning’.

Cherkesov was fired from this role in the wake of his criticism of the secret services under Putin.

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Some interpreted his demise and the poisonings as due to brutal turf wars among Putin’s warring henchmen.

Yet Putin did not want to lose him completely.

Cherkesov – seen by one source as ‘knowing where the bodies are buried over many years’ – was made Head of the Federal Agency for the Supply of Weapons, Military, Special Equipment and Materials for a two year stint.

He later became an MP, but was never again seen as being in Putin’s inner circle, even though the men had been close since university.

Yet he was described as ‘one of the few people with whom Putin is frank’.

Cherkesov was a colonel-general in the reserves and a retired police general (Picture: east2west news)

Cherkesov was deputy head of the FSB at the time of apartment block explosions in September 1999 in the Russian cities of Buynaksk, Moscow and Volgodonsk, in which 300 died, and a near miss in Ryazan, where a device was disarmed.

The FSB blamed the attacks on Chechen terrorists but independent journalists claimed the secret services was behind them in a political ploy to aid Putin.

Putin – then PM – used the ‘terrorist threat’ to wage a ‘popular war’ in Chechnya which helped him secure the presidency for the first time in 2000.

Unlike many other old Putin cronies, Cherkesov was not seen as flamboyantly cashing in on his connections, and was not listed as a billionaire.

Cherkesov was a colonel-general in the reserves and a retired police general.

He was an honoured lawyer and ‘honorary counterintelligence officer’.

His second wife Natalya Chaplina, 65, runs a news agency with close links to the Russian security services.

He had been in charge of the KGB unit responsible for combating anti-Soviet propaganda in the 1980s.

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No further details have been announced after the former Putin ally died of an ‘unexplained disease’