Russia's security agency has opened an espionage case in connection with claims made by Andrei Lugovoy, the suspect named by Britain in the death of former agent Alexander Litvinenko in London last year.
The Federal Security Service, or FSB, did not name any suspects.
Last month, Lugovoy claimed that both Litvinenko and his associate, Kremlin foe Boris Berezovsky, had contacts with Britain's MI6 foreign intelligence agency, and that Berezovsky had given British intelligence sensitive information about Russia.
In a one-sentence statement, the FSB said it had opened a criminal investigation on suspicion of espionage on the basis of statements made by Lugovoy, who met Litvinenko in London on the day he said he fell ill.
Russia has stressed that it will refuse Britain's request for Lugovoy's extradition.
The espionage case is likely to further strain relations between London and Moscow, which is angry over Britain's refusal to hand over Berezovsky and other Kremlin enemies for prosecution in Russia.
It comes against the background of persistent friction between Russia and the West, and repeated Russian claims that British and other Western intelligence agencies - as well as Russians such as Berezovsky who have found refuge abroad - are seeking to weaken Russia.
