Andrei Lugovoi, accused by the U.K. of poisoning former Russian intelligence officer Alexander Litvinenko, alleged at a news conference earlier in the day that Litvinenko and fugitive tycoon Boris Berezovsky worked for the British secret services.
"Information revealed by Lugovoi and Kovtun [another suspect in the case] during a news conference Thursday will be analyzed from a legal standpoint, in particular as there is a suggestion of possible threats to Russia's national security," an FSB spokesman said.
Lugovoi quoted the late Litvinenko as saying the British secret services had recruited him, and then Berezovsky. "Then Berezovsky followed his advice, and handed over some documents from the [Russian] Security Council, which he held as a former deputy council secretary, and became an MI6 agent," he said.
Litvinenko, an associate of Berezovsky, died of radioactive poisoning in London in November.
Lugovoi, who Russia refuses to extradite to the U.K. despite an official request, offered three versions of Litvinenko's poisoning - by the British secret services, the "Russian mafia" and Berezovsky, and said he could prove the British secret services' involvement. "I am very serious about what I am saying, including these accusations," he said.
Lugovoi told reporters that the British secret services had been looking for information to discredit President Putin.
"The British basically proposed that I collect any materials to discredit Vladimir Putin and his family," Lugovoi said.
In addition, he accused Litvinenko of cooperating with Chechen separatist emissary Akhmed Zakayev, who has political refugee status in Britain. Zakayev is wanted in Russia on terrorism charges.
Litvinenko "was in Istanbul on instructions from Akhmed Zakayev, where he met with some representatives of Chechen groups," Lugovoi said.
After the news conference, a spokesman for the Prosecutor General's Office said: "All information on the Litvinenko murder case revealed by Andrei Lugovoi will be thoroughly studied within the framework of the criminal case, which the Russian Prosecutor General's Office is investigating."
The spokesman said Lugovoi's information would be assessed in the light of other widely-publicized criminal cases in Russia, including the murders of two lawmakers, and two journalists, notably Anna Politkovskaya.
The U.K. applied on Monday for the extradition of Lugovoi, saying it had enough evidence to charge him with the murder of Litvinenko.
Russian prosecutors have refused to extradite Lugovoi, saying it was against the Russian Constitution. Moscow, in turn, has also been fruitlessly seeking the extradition of Berezovsky, accused of fraud at home, from Britain where he has been based since 2001 and became a British citizen in 2003.
The British Embassy in Moscow responded Thursday to Lugovoi's allegations by saying the Litvinenko case was a criminal case rather than an intelligence issue.
"Litvinenko's case is a criminal case, not an intelligence matter," the embassy spokesman said. "We expect an official and constructive response."
The spokesman said a British citizen had been murdered, and other Britons and foreigners had been exposed to danger in the polonium poisoning case. The Litvinenko inquiry has discovered traces of polonium-210, the radioactive isotope that killed the former FSB officer, in London, Germany and in planes that flew between Moscow and London.
Later Thursday, British TV channel Sky News cited sources in British intelligence saying MI6 believed Lugovoi's allegations were a "smokescreen" to divert public attention from Britain's request to extradite Lugovoi.
MI6 officials said allegations that British intelligence was responsible for the murder of ex-FSB officer were "ridiculous."