British police investigating the poisoning of a former KGBagent formally requested assistance from their counterparts in Russia, whose foreign minister warned Monday that continued suggestions of Kremlininvolvement in the death could damage diplomatic relations.

Alexander Litvinenkodied Nov. 23 in London after ingesting the radioactive isotope polonium-210. In a deathbed statement, he blamed President Vladimir Putin -- an accusation the Kremlin has vehemently denied.

Radiation has been found at a number of sites in London, as well as on two British Airways planes that traveled the Moscow-London route since Nov. 1, when Litvinenko is believed to have been poisoned. On Monday, British authorities said they were investigating two additional central London locations.

Meanwhile, lawyers for another former security officer -- now in prison in central Russia -- appealed to the British to collect testimony as soon as possible from Mikhail Trepashkin, saying he had key evidence, but his life was in danger.

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In a letter from prison, Trepashkin, who is serving a four-year sentence for revealing state secrets, has said he had warned Litvinenko several years ago about a government-sponsored death squad that intended to kill him and other Kremlin opponents.

"Trepashkin said he had information that could shed light on the killing" of Litvinenko, his lawyer Yelena Liptser told The Associated Press. "If the authorities don't allow him to do that, that would mean they are trying to hide something."

In letters released by his supporters, Trepashkin said an officer of the Federal Security Service, a KGB successor agency known by the acronym FSB, offered him a chance in 2002 to join a group targeting Litvinenko that he rejected. Trepashkin quoted the officer as saying that "Litvinenko won't escape Trotsky's ice pick," referring to the 1940 murder of Leon Trotsky by a Soviet agent.

Foreign Minister Sergey Lavrov said he spoke with British Foreign Secretary Margaret Beckett "about the necessity to avoid any kind of politicization of this matter, this tragedy," the RIA-Novosti news agency reported.

"If the British have questions, then they should be sent via the law enforcement agencies between which there are contacts," he said.

Russia's Prosecutor General's office said it agreed to help Scotland Yard officers in the investigation.

British police arrived in Moscow on Monday and Britain's Home Secretary John Reid said his government "got assurances ... that the Russian government will give us all of the assistance that is necessary."

"The police are on their way to Russia and will go anywhere else where is necessary in order to investigate the circumstances of the suspicious death," Reid told reporters on the sidelines of an EU justice and interior ministers meeting.

Among those to be interviewed is Andrei Lugovoi, another former spy who met Litvinenko on Nov. 1 -- the day he fell ill. FBI agents also have been involved in the probe.

A business associate, Vyacheslav Sokolenko, told The Associated Press on Monday that Lugovoi could not be reached pending his questioning by the British investigators. Sokolenko refused to comment further.

Lavrov denied reports saying Russian diplomats had been instructed to lodge a protest with British authorities over publication of Litvinenko's deathbed accusations.

"Diplomats have not received and could not receive such orders," Lavrov said in Brussels, according to Russian news agencies.

Italian Foreign Minister Massimo D'Alema said he would discuss the case with Putin when the two meet Tuesday in Moscow. An Italian security expert, who met with Litvinenko the day he fell ill, has tested positive for traces of the same radioactive isotope that doctors say killed the Russian agent.

The Italian, Mario Scaramella, has said he showed Litvinenko e-mails suggesting Russian agents' involvement in the October killing of investigative journalist and Kremlin critic Anna Politkovskaya.

Also Monday, the Russian newspaper Kommersant quoted Litvinenko's father as saying he had warned his son his life was in danger.

"He listened to me but, unfortunately, continued to do what he was doing -- exposing his former colleagues," Valter Litvinenko said, according to the newspaper. He told me 'What do you mean, Papa, I'm a British citizen; they would never dare to touch me."'