Scientists discovered more traces of radiation and three people who fell sick were being tested Monday for the deadly radioactive poison that killed former Russian spy Alexander Litvinenko, prompting the British government to appeal for calm.

In a special address to the House of Commons, Home Secretary John Reid said the tests on the three people were only a precaution and pointed out that the radiation caused by polonium-210, the substance doctors say was used to poison the former KGB agent turned Kremlin critic, did not pose a threat to the general public.

"The nature of this radiation is such that it does not travel over long distances, a few centimeters at most, and therefore there is no need for public alarm," Reid said told lawmakers after opposition calls.

Reid warned against rushing to conclusions over who might be responsible for murder of the 43-year-old Litvinenko.

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Ingesting or inhaling even a small amount of polonium-210 — a rare radioactive element usually manufactured in specialized nuclear facilities — can be deadly.

Traces of radiation were found at a bar in London's Millennium Hotel, a branch of Itsu Sushi restaurant near Piccadilly Circus, Litvinenko's house in North London and a section of the hospital where he was treated before he fell ill on Nov. 1.

The sushi restaurant and part of the hospital have been closed for decontamination while tests are still underway to determine if the hotel needs to be decontaminated.

Two other sites — an office block in London's West End and an address in the posh neighborhood of Mayfair — also showed traces of radiation, according to residents, although police and health officials would not immediately confirm this.

Hundreds of people have called a health hotline over concerns they may be at risk but only 18 people were referred to the Health Protection Agency.

Out of those 18, three exhibited symptoms that health officials thought should be examined at a special clinic as a precaution, said Katherine Lewis, a spokeswoman for the Health Protection Agency. She refused to elaborate on their symptoms. The tests should take about a week.

Derek Hill, an expert in radiological science at the University College London, said the public health risk was low.

Although an autopsy has not started yet because of concerns over radioactivity, an inquest into his death could begin as early as Thursday, according to Matt Cornish, a spokesman for Camden Council. The local government body oversees the North London Coroner's Court. The opening is a legal formality, and such inquests are almost always adjourned immediately, sometimes for months.

Coroner's inquests in Britain are meant to determine the cause of death but they sometimes cast blame.

British officials have avoided blaming Moscow for the death of Litvinenko but emergency talks continued Monday over the spy's death — an issue that could overshadow tough negotiations over energy issues and Russia's cooperation on Iran's nuclear ambitions.

In the strongest comments leveled at Moscow since the ex-spy's death, Cabinet minister Peter Hain on Sunday accused Putin of presiding over "huge attacks on individual liberty and on democracy" and acknowledged that relations between London and Moscow were at a difficult stage.

Hain, the government's Northern Ireland secretary, said Putin's tenure had been clouded by incidents "including an extremely murky murder of the senior Russian journalist" Anna Politkovskaya. Litvinenko had been investigating her murder.

Opposition leaders demanded an explanation from the government on how the deadly polonium-210 came to be in Britain.

"All premises in the United Kingdom that use polonium-210 are strictly regulated by the environment agencies," Reid told legislators. "There are some 130 premises that might use that. ... There has been no recent report of the loss or theft of a polonium-210 source in England or Wales."

The ex-spy told police he believed he was poisoned Nov. 1 while investigating the October slaying of Politkovskaya, another critic of Putin's government. The ex-spy was moved to intensive care last week after his hair fell out, his throat became swollen and his immune and nervous systems suffered severe damage.

London's Metropolitan Police said they were investigating it as a "suspicious death" rather than murder. They have not ruled out the possibility that Litvinenko may have poisoned himself.