Mr Litvinenko died in London last month

"Co-operation has already begun," Interpol's Russian office chief Timur Lakhonin said in Moscow.

He said Interpol was providing "speedy exchange of information" between police in Britain, Russia and Germany, who had already launched their own inquiries.

Mr Litvinenko, 43, died in a London hospital last month - it is suspected he was poisoned by polonium-210.

Interpol, which has 186 member countries around the globe, facilitates cross-border police co-operation.

It is hoped that its involvement in the Litvinenko case could speed up the exchange of information between detectives in the three countries involved and also make it easier for them to obtain evidence and testimony.

Contacts questioned

British investigators have already travelled to Moscow to question those who met Mr Litvinenko in a London hotel on 1 November - the day he fell ill.

Andrei Lugovoi has been questioned by police in Moscow

Litvinenko case: Key names Germany gripped by spy story

Andrei Lugovoi - a KGB officer-turned-businessman - was interviewed on Monday in a Moscow hospital where he is undergoing medical checks after traces of polonium-210 had been reportedly found in his body.

Mr Lugovoi - who denies any involvement - said afterwards that he was questioned as a witness.

Dmitry Kovtun - another man who met Mr Litvinenko on 1 November - has also been questioned by British and Russian detectives.

On Tuesday, Mr Kovtun - who has been hospitalised with radiation poisoning in a Moscow hospital - told Russia's Channel One TV that his "health was improving".

The former KGB officer is being investigated by German detectives on suspicion of plutonium smuggling into Germany in October.

But in a separate interview, Mr Kovtun said he had been contaminated by Mr Litvinenko in London weeks before the latter fell fatally ill.

"I have only one explanation for the presence of polonium," Mr Kovtun told Spiegel TV.

"It is that I brought it back from London, where I met Alexander Litvinenko on October 16, 17 and 18," he said.

However, British detectives reportedly found no radiation on a bus ticket Mr Litvinenko used to get to the meeting at the London hotel on 1 November.

If confirmed, this implies that Mr Litvinenko was not contaminated before the meeting.

In Germany, four people close to Mr Kovtun - his ex-wife, her partner and two children living in or near Hamburg - are still undergoing tests for possible radiation contamination.

According to German police, the four have shown signs of contamination after Mr Kovtun stayed at his ex-wife's flat before flying to London.

He denies any involvement in Mr Litvinenko's death.

'Whistleblower missing'

Russian investigators, meanwhile, are reportedly preparing to travel to London to question witnesses.

Mr Litvinenko's widow earlier said she would not assist the Russian inquiry.

Friends of Mr Litvinenko believe he was poisoned because of his criticisms of the Russian government since defecting to the UK, but the Kremlin has dismissed suggestions it was involved in any way.

Russia's foreign intelligence service has also issued a statement denying any involvement.