MOSCOW - President Vladimir Putin leads Russians in a day of mourning on Monday with his people asking how more than 100 hostages were killed by mystery gas used to knock out Chechen guerrillas holding them in a Moscow theater.

As of Sunday evening, Moscow's top doctor Andrei Seltsovsky said 646 people were still in hospital, including 150 in intensive care, 45 of them in a grave condition.

Early reports on Saturday said only around 10 hostages had died, suggesting that the operation had been more successful than had first seemed possible.

But the death toll rose inexorably. During Saturday it hit 67, then over 90, before reaching 117 on Sunday. Only two died from gunshots. Asked what had killed the other 115, Seltsovsky said succinctly: "The effects of the gas exposure."

The injuries and deaths followed the seizure of a Moscow concert hall by armed Chechen rebels, bent on forcing Russia to withdraw from their republic, on Wednesday.

For President Vladimir Putin, the rising death toll was an uncomfortable reminder of two other tragedies which have blighted his term of office.

In August 2000, the nuclear submarine Kursk sunk after a torpedo exploded on board, killing 118. Putin was widely criticized for a perceived failure to act decisively then.

Earlier this year a helicopter was shot down over Chechnya with a similar number of deaths, despite Putin's repeated claims that the Chechen war, launched in September 1999 -- months ahead of the election which made him president -- was all but over.

ALFA TROOPS

After the guerrillas had shot two hostages dead, one of them early on Saturday, crack Alfa troops stormed the theater, first pumping in an unknown gas which knocked out both hostages and rebels.

Officials said they were forced to use the gas to stop several women fighters seated among the 750 hostages detonating explosives strapped to their waists.

The storm troops then took on the remaining rebels in the rooms and corridors of the second floor in close-quarter combat.

Putin apologized and asked for forgiveness for the deaths, but televised scenes of desperate relatives clamoring outside hospitals for news of their loved ones and being pushed back by police did little to boost his image.

A failure to identify the gas -- and claims that it was a drug similar to general anesthetics used in routine surgery -- also helped reinforce a long-standing image of Russian secrecy and disinformation. Dr. Peter Hutton, head of Britain's Royal College of Anesthetists, said he knew of no medical anaesthetic gas that could have been used in the way the gas was used in Moscow.

"It's almost certainly something that's developed, owned, and used only by the military," he said.

Paul Beaver, of the London-based security and defense consultancy Ashbourne Beaver Associates, said the operation would be considered a success in military terms, defined as fewer than 30 percent casualties.

But he said most military gases have antidotes and it may have been a flaw in Russian planning that they launched their attack without making sure they had enough antidote on hand to treat all the hostages for poisoning.

He was backed up by Lev Fyodorov, president of a Russian chemicals security pressure group, who said troops failed to give an antidote to those affected by gas when they were still in the theater, or once they had dragged them out onto the street, or even when they got them to hospital.

Moscow's top anesthetist, Yevgeny Yevdokimov, made clear that doctors had been hampered by the fact that they did not know what gas they were dealing with.

Russia now faces possible strains in relations with its partners as it continues to portray its Chechen policy as an anti-terrorist operation along the lines of the U.S.-led coalition against the al Qaeda network in Afghanistan.

On Sunday the foreign ministry summoned the French ambassador to complain about a pro-Chechen rally held in Paris on Saturday.

The ambassador, Claude Blanchemaison, was also told Russia was deeply concerned that Denmark, which holds the rotating Presidency of the European Union, had not denounced a planned World Congress of Chechens in Copenhagen.

Moscow has threatened Putin may cancel a forthcoming trip to Denmark if the gathering went ahead.