Scotland Yard officers, assisted by the FBI, have already traveled to Virginia to interview former KGB officer Yuri Shvets.

A United States-based friend of the former Russian spy Litvinenko said he had given police the name of a suspect he believes orchestrated his killing, according to Associated Press' report.

Shvets said he had known Litvinenko since 2002 and had spoken to him on Nov. 23, the day he died.

"The truth is, we have an act of international terrorism on our hands. I happen to believe I know who is behind the death of my friend Sasha (Litvinenko) and the reason for his murder," Yuri Shvets said in an exclusive interview with the AP by telephone from the United States.

"This is firsthand information, this is not gossip. I gave them the firsthand information that I have," Shvets told the AP.

"I want this inquiry to get to the bottom of it, otherwise they will be killing people all over the world - in London, in Washington and in other places," Shvets said.

Moreover, police sources confirmed that counter-terrorism officers were expected to leave for Moscow "very soon", though a spokeswoman at Scotland Yard, the headquarters of London's Metropolitan Police, declined to comment.

The BBC said nine officers could travel to Russia as early as Monday.

A report in the News of the World newspaper, Britain's biggest-selling weekly tabloid, said they were to interview the three Russians who met Litvinenko on Nov. 1, shortly before he fell ill.

"He said I want to be buried according to Islamic tradition. I said okay son. It will be as you wish. We already have one Muslim in our family. The important thing is to believe in the Almighty. God is one," he said in the interview. Valter Litvinenko said he was sure that President Vladimir Putin was involved in the death a view apparently voiced by the former agent himself in a letter before his November 23 death and rejected claims that former Russian agents may have been responsible, the AFP news agency reports.

"No kind of veterans organization would dare to kill a former secret service member. There was an order right from the top to kill my son," Valter Litvinenko said. "I am in no doubt that this was done by members of the Russian secret services, with the permission of Vladimir Putin," he said.

Russian authorities have denied any involvement in Litvinenko's death.

The father added that he would return to Russia after burying his son, although he considered that returning was not without danger.

Separately, Russia's foreign ministry said on Monday that British investigators had been issued with visas to come to Russia to pursue their inquiries into Litvinenko's death.

"The police have left today for Moscow to carry out enquiries," British Home Secretary John Reid told journalists in Brussels on the margins of a meeting of European Union justice and home affairs ministers.

Lavrov, also in the Belgian capital, said the fallout from Alexander Litvinenko's death was harming relations with Britain. Litvinenko died in a London hospital on November 23, three weeks after suffering radiation poisoning caused by Polonium 210.

Lavrov said insinuations in Britain of high-level Russian involvement in Litvinenko's death were "unacceptable," adding: "It is of course damaging our relations." Both Russia and Britain believe Litvinenko's death should not be politicized, Lavrov added. "If there are any questions, they should be put through law enforcement agencies," Interfax quoted him as saying.

Responding to Lavrov's comment, Reid said British Foreign Minister Margaret Beckett had been in touch with Moscow "and they have assured us we'll get all the cooperation necessary." Asked how long the British police would spend in Russia, a British embassy spokesman said "They'll stay as long as it takes."

Associates of Litvinenko have alleged either Kremlin involvement in his killing or that rogue elements in Russia's state security service were responsible.

Before he died Litvinenko, a former Russian state security service agent who became one of President Vladimir Putin's sharpest critics in the London-based Russian emigre community, accused Putin of ordering his death.

The Kremlin has strongly denied any part in the killing, and Kremlin opponents also find the theory of Putin's involvement highly improbable, noting such a high-profile killing on foreign soil could only damage him.

Italy's foreign minister said he would ask Putin in Moscow on Tuesday to help the British police in their investigation. "I think that it is also an opportunity, given that Putin has decided to receive me, to tell Russian authorities that we want answers," Massimo D'Alema told reporters in Belgrade. "It's clear that I will ask Russia to offer its full cooperation to the judiciary and to the British police forces above all."

Scotland Yard detectives are likely to try to interview Russian citizens who met Litvinenko at London's Millennium Hotel on November 1, the day he fell ill.

Andrei Lugovoi, a former KGB agent, says he and businessman Dmitry Kovtun met Litvinenko that day at the hotel. But Lugovoi, now back in Moscow, says they discussed a business opportunity and denies anything to do with an attempt on Litvinenko's life.

Alex Goldfarb, a London-based friend of Litvinenko, said the British investigators should insist on also seeing another ex-KGB agent, Mikhail Trepashkin, who had "substantive information" of use to them. Trepashkin, serving a four-year sentence in an Urals prison for divulging state secrets, alleged in a letter last Friday that the FSB, the Russian state security service, had created a hit squad to kill Litvinenko and other enemies of the Kremlin.

"He [Trepashkin] says that he possesses information that may shed light on the murder. And he is ready to speak out," Yelena Liptser, a lawyer for Trepashkin, told a news conference in Moscow on Monday. Trepashkin told his lawyers about his readiness to testify in Nizhny Tagil, where he is serving his sentence, she said.

Russia on Monday cleared the way for British detectives probing the mystery poisoning of former KGB spy Alexander Litvinenko to visit Moscow, and promised to help their investigation, the Reuters news agency reported Monday.

Friends of Litvinenko, who died from radioactive poisoning from polonium 210, said meanwhile that the British investigators should insist while in Russia on seeing Mikhail Trepashkin, who had "substantive information" of use to them.

Before he died, Litvinenko accused Russian President Vladimir Putin of ordering his slow and agonizing death, which sparked a health scare in Britain and strained London's relations with Moscow.

The Kremlin has denied any part in the death of the former Russian state security service agent who became one of Putin's sharpest critics in the London-based Russian emigre community.

The foreign ministry in Moscow confirmed on Monday that visas has been issued to Scotland Yard investigators and the Russia's general prosecutor's office offered to cooperate.

While Litvinenko blamed Putin for his death, other theories have centered on the possible involvement of rogue Russian agents, acting independently of the Kremlin.

Litvinenko met Russian citizens at London's Millennium Hotel on Nov. 1, the day he fell ill.

Andrei Lugovoi, also a former KGB agent, agrees he and businessman Dmitry Kovtun met Litvinenko that day at the hotel. But Lugovoi now back in Moscow, says they discussed a business opportunity and he denies anything to do with an attempt on Litvinenko's life.

In Brussels, British Home Secretary (interior minister) John Reid said the police investigation would push forward irrespective of whatever diplomatic fall-out there might be. "Police will issue their own bulletin, but they will be going to Russia," he told journalists after arriving for a meeting of European Union justice and home affairs ministers.

"This investigation will proceed as normal whatever the diplomatic or whatever the wider considerations," he said. Reid said he would seek to reassure his EU colleagues that the health threat arising from traces of polonium 210 found in several locations in London was minimal.