Friends of former Russian spy Alexander Litvinenko accused "evil forces" in Russia on Friday of being responsible for his death in London and said the poisoning of the ex-colonel was an act of revenge.

Russia said it was "silly" to suggest the Kremlin had orchestrated a plot against 43-year-old Litvinenko. British doctors said Litvinenko had been poisoned but did not know with what.

Litvinenko, who lost his hair and suffered organ failure, died in a London hospital late on Thursday, three weeks after being poisoned.

"He was fighting against the evil forces in Russia, against the KGB, against the authorities which are suppressing democracy and liberal freedoms in Russia," Oleg Gordievsky, a friend of Litvinenko, told Sky television.

"He became a victim of ... revenge and malice of those forces in Russia," said Gordievsky, also a former Russian agent who defected to Britain.

One doctor said the type of poison used may never be known. Medical experts ruled out earlier suggestions that it was a heavy metal such as thallium or a radioactive substance. British police said they were investigating Litvinenko's "unexplained" death. He became a British citizen on October 1.

"The bastards got me. But they won't get everybody," Litvinenko told friend and filmmaker Andrei Nekrasov earlier this week, according to Friday's Times newspaper.

Litvinenko, who fled to Britain in 2000 with his wife and son and was granted asylum, said he fell ill after meeting two Russians at a hotel. He had been investigating the killing of Russian journalist Anna Politkovskaya, who was gunned down at her Moscow flat on October 7.

A source in the Russian delegation at EU-Russia summit in Helsinki told reporters "It is a human tragedy. The man was poisoned. But the accusations towards the Kremlin are so unbelievable, they are too silly to be commented on by the president or anyone from the Russian side."

Litvinenko served in the KGB's counter-intelligence department and then the FSB's highly secret organized crime group. The FSB is the main successor organisation to the Soviet KGB and deals with internal threats.

Source: China Daily