A Kremlin spokesman has promised a "targeted" response to Britain's expulsion of four Russian diplomats, raising the stakes in a dispute over the murder probe of a former KGB spy.

Russian deputy foreign minister Alexander Grushko briefed reporters in Moscow Tuesday, a day after Britain announced the explusions. Grushko told the Itar-Tass news agency to anticipate the Russian government's response to Britain's "provocative" move "in the very near future."

Grushko said it would be a "targeted and appropriate" response, but would not give further details other than to add that Russia's action would take into account "the interests of ordinary people and businessmen."

Analysts have said it is likely Russia will reciprocate by evicting British diplomats in Russia, but that they will stop short of hitting back at British business interests in Russia — a move that could be costly for both countries.

Meanwhile in London, a spokesman with the Foreign Office told reporters that "no reprisal on behalf of Russia would be justified."

The diplomatic row is rooted in the British investigation into the slaying of former KGB spy and fierce Kremlin critic Alexander Litvinenko, who was poisoned by the radioactive isotope polonium-210 while dining at a London hotel on Nov. 1, 2006.

British prosecutors believe that Andrei Lugovoi, another former KGB agent who met with Litvinenko that day, is a prime suspect and they want him extradited to stand trial in London.

But Russia has refused to co-operate, arguing that it is against its constitution to extradite citizens wanted for crimes in foreign nations.

In retaliation, Britain's Foreign Ministry announced Monday that it was ordering four Russian diplomats to leave London to show the country is serious about prosecuting Lugovoi for what British Foreign Secretary David Miliband called a "heinous" crime.

"This response is proportional and it is clear at whom it is aimed," Miliband said Monday.

Britain has reportedly sent a list of four names threatened with expulsions to Russian officials, but Russia said it would wait for Britain to follow through with the expulsions before acting.