Russia's security agency has opened an espionage case in connection with claims made by the suspect named by Britain in the radiation poisoning death of former agent Alexander Litvinenko in London last year.
Last month, suspect Andrei Lugovoy claimed that both Mr Litvinenko and his associate, Kremlin foe Boris Berezovsky, had contacts with Britain's MI6 foreign intelligence agency, and that Mr Berezovsky had given British intelligence sensitive information about Russia.
In a one-sentence statement, Russia's Federal Security Service (FSB) said it had opened a criminal investigation on suspicion of espionage on the basis of statements made by Mr Lugovoy, who met with Mr Litvinenko in London on the day he said he fell ill.
Russia has said it will refuse Britain's request for the extradition of Mr Lugovoy.
The espionage case is likely to further strain relations between London and Moscow, which is angry over Britain's refusal to hand over Mr Berezovsky and other Kremlin foes 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 Mr Berezovsky who have found refuge abroad - are seeking to weaken Russia.
AP