Russia was tracking the assassins of dissident spy Alexander Litvinenko before he was poisoned but was warned off by Britain, which said the situation was "under control", according to claims made in a leaked US diplomatic cable.

The secret memo, recording a 2006 meeting between an ex-CIA bureau chief and a former KGB officer, is set to reignite the diplomatic row surrounding Litvinenko's unsolved murder that year, which many espionage experts have linked directly to the Kremlin.

The latest WikiLeaks release comes after relations between Moscow and London soured as a result of Britain's decision to expel a Russian parliamentary researcher suspected of being a spy.

The memo, written by staff at the US embassy in Paris, records "an amicable 7 December dinner meeting with ambassador-at-large Henry Crumpton [and] Russian special presidential representative Anatoliy Safonov", two weeks after Litvinenko's death from polonium poisoning had triggered an international hunt for his killers.

During the dinner, Crumpton, who ran the CIA's Afghanistan operations before becoming the US ambassador for counter-terrorism, and Safonov, an ex-KGB colonel-general, discussed ways the two countries could work together to tackle terrorism. The memo records that "Safonov opened the meeting by expressing his appreciation for US/Russian co-operative efforts thus far. He cited the recent events in London – specifically the murder of a former Russian spy by exposure to radioactive agents – as evidence of how great the threat remained and how much more there was to do on the co-operative front."

The memo contains an observation from US embassy officials that Safonov's comments suggested Russia "was not involved in the killing, although Safonov did not offer any further explanation".

Later the memo records that Safonov claimed that "Russian authorities in London had known about and followed individuals moving radioactive substances into the city but were told by the British that they were under control before the poisoning took place".

The claim will be rejected in many quarters as a clumsy attempt by Moscow to deflect accusations that its agents were involved in the assassination.

Russia says it had nothing to do with the murder, but espionage experts claim the killing would not have been possible without Kremlin backing. Shortly before he died, Litvinenko said he had met two former KGB agents, Dmitry Kovtun and Andrei Lugovoi, on the day he fell ill. Both men deny wrongdoing, but Britain has made a formal request for Lugovoi's extradition following a recommendation by the director of public prosecutions.

New evidence linking Russia with the death of Litvinenko was recently produced by his widow, Marina, who procured documents allegedly showing the FSB security service seized a container of polonium in the weeks before the poisoning. Moscow disputes the claims.

The allegation that British authorities were monitoring the assassins' progress through London is likely to raise questions about whether Litvinenko was warned his life may have been at risk in the days before he was murdered.

Several people familiar with the affair said they thought Safonov's claims implausible, with one saying he had never heard it aired within London intelligence circles before. Nevertheless Safonov's remarks – in effect questioning the competence of Britain's security services – will do little to heal the relationship between London and Moscow.

The claims come after Britain announced that Katia Zatuliveter, a 25-year-old Russian working for the Liberal Democrat MP Mike Hancock, is to be deported amid suspicions she was spying for the Kremlin, a charge she plans to contest.

Alexander Sternik, chargé d'affaires at Russia's embassy in London, hinted that the deportation could trigger tit-for-tat expulsions and denounced the move as a "PR stunt" designed to mask Britain's own problems. "These problems are many over the last couple of months," Sternik said. "You can cite the unflattering leaks from WikiLeaks and [England's] unsuccessful [World Cup] bid."

The Paris embassy memo also shines new light on relations between Washington and Moscow. Henry Crumpton reportedly gained almost mythical status after the 9/11 terrorist attacks. He has been identified in the US media as a CIA agent quoted in the 11 September commission report as unsuccessfully pressing the agency to do more in Afghanistan to combat Osama bin Laden.

Safonov was once tipped to take the top job at the federal security service after the then Russian president, Boris Yeltsin, dismissed its incumbent.