MOSCOW - Russia on Tuesday vowed a "targeted and appropriate" response to Britain's expulsion of four diplomats in a mounting confrontation over the probe into the radiation poisoning death of a former KGB officer.

Deputy Foreign Minister Alexander Grushko criticized Britain for demanding the extradition of the key suspect in the killing of Alexander Litvinenko, emphasizing that Russia's constitituion forbids it, and urged other European nations not to gang up against the Kremlin.

The dispute could derail recent American efforts to ease increasing tension between Russia and the West, and holds implications for crucial issues like energy security and the nuclear standoff with Iran.

Britain decided to throw out four diplomats and place restrictions on visas for Russian officials in response to the Kremlin's refusal to hand over Andrei Lugovoi for prosecution the November killing of Litvinenko in London.

Grushko said Russia would inform Britain very soon of its response, but did not say what it would be. Prime Minister Gordon Brown's spokesman said any response was up to the Russians.

"Our reaction will be targeted and appropriate, and the British authorities will be officially informed of this in the nearest future," Grushko told reporters. He said Moscow would take care that tourists, business people and other "everyday citizens" with ties to Britain would not be affected.

As President Vladimir Putin and other have done, Grushko railed against Britain for demanding Lugovoi's extradition, accusing Britain of "trying to punish (Russia) for abiding by its own consitution."

Grushko indicated Russia might not respond with a tit-for-tat expulsion of diplomats, stressing that if Moscow were to expel four people for every one that Britain has refused to hand over to Russia, "the British embassy would today be lacking 80 employees."

He said that Mosocw has sought the extradition of 21 people for prosecution in Russia, including Boris Berezovsky -- a vocal Putin opponent and associate of Litvinenko -- and the Chechen rebel figure Akhmed Zakayev, both of whom have refugee status in Britain.

"Not one has been handed over," Grushko said.

In another hint at a possible response, Grushko said that "the line London has taken will complicate, if not make impossible, the cooperation of law enforcement organization on questions that touch the security interests of millions of Britons and Russians" -- an apparent warning Moscow could reduce anti-terror cooperation.

Viktor Kremenyuk, deputy director of the Institute of USA and Canada Studies of the Russian Academy of Sciences, said he would advise the Kremlin not to escalate the conflict by responding with more expulsions or other sanctions. He also questioned why the British government has taken the case so far.

"I cannot understand why the British government has decided to turn this more or less usual criminal case into a political one," Kremenyuk said in an interview. "Why does one former KGB officer killing another deserve this attention?"

Lugovoi, also a former KGB officer, gave a television interview Tuesday in which he said Britain had provoked the diplomatic conflict.

"The British officials have backed themselves into a corner by proposing to extradite me, although it was obviously prohibited by the (Russian) Constitution," he said on Russia Today, an English-language satellite channel under Kremlin control.

Izvestia, a daily newspaper loyal to the Kremlin, announced in a front-page headline that "the new British prime minister has declared a diplomatic and visa war on Russia."

Nezavisimaya Gazeta said that British investment in Russia in 2006 amounted to US$3.3 billion and was expected to double this year. "Looks like Russia and Britain have something to lose," the paper said.