Former Russian Prime Minister Yegor Gaidar may have been poisoned by an unnatural source after becoming violently ill and fainting during a trip to Ireland, doctors said.

Gaidar, a former premier and head of a liberal opposition party, was rushed to a hospital after falling unconscious during a conference in Ireland. On Thursday he was in stable condition at a Moscow medical facility.

"So far none of the doctors who have examined me can explain the reasons for what had happened to me," Gaidar told the Financial Times.

Medical experts in Ireland were seeking more information on his condition immediately after he became sick.

"Doctors don't see a natural reason for the poisoning and they have not been able to detect any natural substance known to them" in Gaidar's body, spokesman Valery Natarov said. "So obviously we're talking about poisoning [and] it was not natural poisoning."

Ekaterina Genieva, a conference organizer, described Gaidar as pale before becoming ill.

"I rushed after him and found him lying on the floor, unconscious. He was vomiting blood and also bleeding from the nose for about 35 minutes," Ms Genieva told the Financial Times.

Gaidar, 50, one of the leaders of a liberal opposition party who served briefly as prime minister in the 1990s under President Boris Yeltsin. His illness follows the poisoning of former KGB spy Alexander Litvinenko, who died in London just one day before Gaidar fell ill. Another former KGB spy who met with Litvinenko on the day he was allegedly poisoned, Andrei Lugovoy, a former KGB colleague who served as Gaidar's bodyguard at one point.

On the day he first felt ill, Litvinenko said he had two meetings. In the morning, he met with an unidentified Russian and with Lugovoy at a London hotel. Later, he dined with Italian security expert Mario Scaramella to discuss the October murder of Politkovskaya.

Scaramella told reporters in Rome on Tuesday that he had traveled to meet Litvinenko to discuss an e-mail he received from a source naming the killers of Politkovskaya, who was gunned down Oct. 7 at her Moscow apartment building, and outlining that he and Litvinenko were on a hit list.

Anatoly Chubais, a top Yeltsin-era government official and now head of the national electricity monopoly, said Wednesday that he suspected a link among Gaidar's illness, Litvinenko's death and last month's murder of investigative reporter Anna Politkovskaya.

Of Gaidar, Natarov said: "His condition is stable and improving. Doctors say there is no threat to his life at the moment."

Colm Keane, spokesman for National University of Ireland at Maynooth, where the conference was held, said Wednesday that medics initially suspected Gaidar's diabetes or some sort of ailment caused his illness.

A spokesman for the Irish Department of Foreign Affairs, speaking on customary condition of anonymity, said "we have received no evidence of anything untoward about this. He was certainly well enough to travel back home."

Gaidar, an economist, is best known for as the architect of the sweeping free-market reforms that were instituted in the early years of former President Boris Yeltsin's administration. He is one of the leaders of the liberal opposition party Union of Right Forces and heads a think-tank called the Institute for the Economy in Transition.

Maria Gaidar, his daughter, is a well-known liberal youth activist and vociferous Kremlin critic.

The Associated Press contributed to this report.