Russian officials refused to give any details yesterday on a mysterious gas used to end a Moscow theatre siege in which at least 118 hostages died, raising questions about whether the gas had killed them.

The Kremlin was silent about a television report that Russian special forces who stormed the theatre in the early hours of Saturday had killed hostages when they used the sleep-inducing gas.

However later yesterday Moscow's top doctor said the gas used by Russian special forces to end the siege and knock out Chechen rebels holding hundreds of people captive, killed 115 hostages.

Moscow's chief anaesthesiologist, Yevgeny Yevdokimov, said the gas used was a "narcotic substance basically similar to a general anaesthetic in surgery".

"As you increase the dose, then naturally bodily disfunctions occur," Yevdokimov told the news conference.

"First of all comes loss of consciousness, then problems with breathing and blood circulation," he said, adding that he did not know the name of the gas.

There was no immediate reaction from the Kremlin to the doctors' statement.

NTV television reported two hostages - a Dutch national and a Kazakh national - had died from gas poisoning.

The unidentified chemical was so powerful that the Chechen suicide fighters who had been filmed during the siege toying with detonators attached to explosives strapped to their waists had no time to set them off.

There have been suggestions that the troops used nerve gas. Sergei, 36, who declined to give his family name, told Reuters after he was released from hospital that the gas had smelled slightly bitter. Chemical warfare experts say nerve gas often smells of bitter almonds.

London-based security expert, Michael Yardley, said he believed the gas used was BZ, a colourless, odourless incapacitant with hallucinogenic properties, first used by the United States in Vietnam.

He said the symptoms displayed by the hostages in Moscow - inability to walk, memory loss, fainting, heartbeat irregularities, sickness - all pointed to BZ. According to the US army the side effects last 60 hours, Yardley said.

"The Russians wouldn't want a big shout about it because it (BZ) is just the sort of stuff they are not supposed to have," he said. "It's not specifically banned, but... it is in a sort of grey area."

Film taken after the special forces stormed the theatre to free more than 750 hostages showed a woman slumped back on a chair with her mouth wide open, and a bag of explosives tied to her front.

"A panic went up among us and people were screaming, 'Gas! gas!' and, yes, there was shooting," theatre director Georgi Vasilev, one of the hostages, told Reuters.

"But then everyone fell quickly down. And then, I was told by one woman while we were in hospital together, but who didn't fall asleep immediately because she covered her mouth and nose, that it was very strange to look at everyone.

"You see, when the shooting began, they (the rebels) told us to lean forward in the theatre seats and cover our heads behind the seats. But then everyone fell asleep. And they (the rebels) were sitting there with their heads thrown back and their mouths wide open."

Though the government says it freed over 750 hostages it has not given any information on how many were hospitalised nor how many were affected by the gas.

One hostage told Interfax news agency that he saw the guerillas convulse and slump because of gas.

"After the first shots at the hostages gas came in, I saw how a terrorist sitting at the scene jumped up and tried to get a respirator. I saw how he convulsed and tried to put the mask to his face and then fell," the unidentified witness said.

A Health Ministry official quoted by Interfax confirmed that 118 hostages had died, with 50, or nearly all of, their captors.

Earlier, Russian officials said the gas was the "special means" to stop the guerillas from blowing up the theatre. The guerillas had threatened to start killing hostages if Moscow did not withdraw its troops from their homeland.

In Moscow, distraught relatives begged for information on loved ones, and police checked cars, passengers and luggage to prevent a feared repeat attack.

Police said they could not immediately confirm a report yesterday that police in Moscow had charged three Chechens with being involved in preparations for the hostage-taking.

Local television said police detained a Chechen woman who had been in hospital on suspicion of involvement in the attack.

The heightened security meant that many Russians could not get in to see relatives being treated in hospitals.

"They carried out the operation to the end, but they haven't worked out what to do for the relatives. Many cannot find their family members," said Anatoly Belayusov, whose 28-year-old daughter Lyuybov was missing after the siege.

President Vladimir Putin asked for forgiveness from the relatives of the dead.

He declared today a national day of mourning as dozens of sympathisers left flowers and cards on a low wall near the theatre. Officers in camouflage and with sniffer dogs entered the building to check for booby traps.

"I would like to address primarily the relatives and friends of those killed. We could not save everyone," Putin said in a television broadcast late on Saturday. "Please forgive us."

Looking exhausted, Putin called on Russians to rebuild their confidence in a country which had dealt with "armed scum".

On Wednesday night, the guerillas burst into a Moscow musical theatre and vowed to kill audience members and staff if Russia failed to pull out troops that returned to Chechnya three years ago on Putin's orders.

Chechnya's fugitive rebel president, Aslan Maskhadov, condemned the siege, saying he rejected "terror as a method of reaching any goals".

A senior aide to Maskhadov said the drama meant Moscow had to choose between talking to gunmen or the man elected president of the breakaway North Caucasus republic in 1997. He warned there could be more such attacks.