US spies intercepted communications between the chief suspects in the murder case of Russian former spy Alexander Litvinenko, linking his poisoning to the Russian state, Britain's Daily Telegraph reported Saturday.

According to the report, the National Security Agency (NSA) obtained electronic messages sent between London and Moscow shortly after the Kremlin critic was poisoned with radioactive polonium-210 while drinking tea at a hotel in the British capital in 2006.

British authorities received the evidence, which is reported to directly implicate the Kremlin, but it is inadmissible in court, the Telegraph said.

However, Litvinenko's widow Marina has applied to the NSA to disclose the intercepts, saying they should be made available to former British judge Robert Owen, who is chairing a nine-week inquiry into the murder that begins in London's High Court on Tuesday, the Telegraph reported.

Litvinenko, 43, an ex-agent in Russia's FSB intelligence agency who turned against his former masters, said he believed Russian President Vladimir Putin was involved in his killing after he publicly criticised the leader, himself an ex-Soviet KGB agent.

British police have identified Russian spy-turned-lawmaker Andrei Lugovoi as the chief suspect and have issued an arrest warrant for his fellow former agent Dmitri Kovtun, but Moscow has refused to hand them over. They both deny involvement.

Marina Litvinenko asked for "NSA intercepts of telephone communications of Mr Andrei Lugovoi and Mr Dimitry Kovtun from London, UK, in the period October 15 to November 1, 2006," in a Freedom of Information request issued last year.

Senior NSA official Paul Blaskowski replied that the NSA could not reveal "existence or non-existence" of the information, the British paper reported.