The US embassy in Moscow says it has identified the gas used by Russian security forces as they stormed a theatre where 800 hostages were held by armed Chechens.

The sleep-inducing substance was not a nerve gas as previously suspected, but an opiate which dulls the senses, the embassy said.

At least 113 of the hostages who died in the theatre are believed to have been poisoned by the gas.

A further four died of gunshot wounds, according to the Moscow prosecutor's office.

Two are known to have been killed by the rebels, but it is unclear whether the other two were killed by the hostage-takers or by Russian gunfire during Saturday's rescue mission.

Most of the 50 hostage-takers, who were demanding Russian troops withdraw from the breakaway republic of Chechnya, were shot during the rescue mission, prosecutor Mikhail Avdykov said on Tuesday.

The use of the gas has been widely condemned as heavy handed, but Moscow insists it had little room for manoeuvre - faced with the prospect of so many rebels prepared to kill themselves and their hostages.

Medicine administered

But the authorities refusal to name the type of gas used by its special forces as they tried to incapacitate the heavily armed guerrillas has been particularly criticised.

BBC science reporter Corinne Podger says that ambulance crews who treated hostages at the theatre were ordered to administer naloxone, a medicine routinely given to patients who have overdosed on opiates such as heroin.

Doctors who have been treating some of the foreign victims have also said that they believe the substance was an opiate.

In too heavy a dose, an opiate can cause coma and death by shutting down breathing and circulation.

Russian officials have cited security reasons for refusing to reveal the kind of gas used.

Some of those who died will be buried in Moscow later on Tuesday.

Scores of the former hostages remain in hospital, some of them in a critical condition. Their relatives are being prevented from visiting them.

Foreign support

The BBC's Jonathan Charles, in Moscow, says the high death toll continues to make the raid controversial, but the strategy is now being studied by foreign intelligence agencies.

They are examining whether the deployment of gas might prove helpful if they are ever faced with similar sieges.

The White House, for its part, has firmly pinned the blame for the civilian deaths on the hostage takers.

President George W Bush "understands it is the terrorists with whom the blame lies", White House spokesman Ari Fleischer said.

Russian President Vladimir Putin's tough line was also endorsed by the UK's prime minister on Monday who said there were "no easy, risk-free, safe solutions" in such a situation.

"I hope people will understand the enormity of the dilemma facing President Putin as he weighed what to do, in both trying to end the siege with minimum loss of life and recognising the dangers of doing anything that conceded to this latest outrage of terrorism from Chechnya," Tony Blair told Parliament in London.

Despite its support for President Putin's policy on the rebels, the United States has, along with other foreign states, asked Russia to explain how so many hostages died from the gas.

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Moscow Mayor Yuri Luzhkov honoured the victims