Russia and Britain continued their war-of-words Wednesday over the London murder of an ex-Russian spy, with Moscow saying it is "very surprised" by Britain's reaction to its refusal to extradite a top suspect.

A Russian foreign ministry spokesman said the country's constitution prohibits the extradition of citizens to foreign states - a point that Russian authorities have made repeatedly in recent weeks.

A British government spokesman voiced disappointment Tuesday at Moscow's refusal to hand over Andrei Lugovoi - London's prime suspect in the 2006 murder of former Russian spy Alexander Litvinenko.

British medical authorities say Litvinenko died of radiation poisoning, and Scotland Yard has linked Lugovoi to the death.

Moscow authorities have proposed launching their own investigation of Lugovoi, but British prosecutors have questioned whether such a probe would be impartial and fair.

Separately Wednesday, Lugovoi accused Britain of trying to cover up poor detective work by complaining about Moscow's refusal to extradite him.

In a letter on his deathbed, Litvinenko accused Russian President Vladimir Putin of ordering his poisoning. The Kremlin denies any involvement in his death.

Some information for this report was provided by AFP and Reuters.