COPENHAGEN - Denmark arrested a senior Chechen rebel on Wednesday at the behest of Russia, which said it suspected him of helping plot last week's Moscow theatre siege in which at least 119 hostages died.

A Danish judge ordered Akhmed Zakayev detained for 13 days, to make sure he did not flee. The justice minister said he might be extradited if Russia promised not to use the death penalty.

Zakayev, a top aide to the region's fugitive separatist president, Aslan Maskhadov, had attended a long-planned Chechen exiles' gathering this week in Copenhagen. Denmark's agreement to allow the meeting enraged Russian President Vladimir Putin, who cancelled a state visit next month in protest.

Police said they received information from Russia on Tuesday that Zakayev was suspected of helping prepare the theatre siege and of taking part in other "terrorist" acts from 1996-99.

"The Copenhagen police found this was sufficient foundation for the arrest," they said in a statement.

Maskhadov and Zakayev have both condemned the raid in Moscow as the work of an extremist rebel faction outside their control.

Prime Minister Anders Fogh Rasmussen denied Denmark had given in to political pressure from Moscow: "This case has been treated according to normal police procedures," he told a news conference. "I would find it very alarming if a government or an opposition interfered in the work of the police."

Russian officials were elated by the detention of Zakayev, the most prominent spokesman for Maskhadov, who was elected in 1997 after forcing out Russian troops but went into hiding in 1999 when Putin sent the army back into the chaotic region.

Zakayev told Reuters in an interview on Sunday he had no link to the three-day siege that began when some 50 Muslim rebels seized a theatre and threatened to kill their more than 750 hostages unless the Russian forces quit their homeland.

DEATH TOLL RISES

Two more captives died overnight, taking the toll to 119, Moscow health officials said. Most of the Chechens were killed.

Funerals were held on Wednesday for two children, actors in the musical "Nord-Ost" which had been playing in the theatre.

A mystery gas used to knock the rebels out when troops stormed in on Saturday led to all but two of the hostage deaths.

Russia refused for four days to name the gas, saying this would aid rebels, but Health Minister Yuri Shevchenko did so on Wednesday, insisting it was not a banned substance.

"To neutralise the terrorists a substance based on Fentanyl derivatives was used," Interfax news agency quoted him as telling told a news briefing.

"I officially declare: chemical substances which might have fallen under the jurisdiction of the international convention on banning chemical weapons were not used."

Fentanyl is a potent opium-based narcotic used as an anaesthetic. Opiates affect pain receptors and cause sleepiness and, in high doses, can lead to respiratory failure.

The U.S. ambassador and others say lives could have been saved if the Kremlin - which did not even tell doctors treating the sick what the gas was - had been less secretive.

A total of 245 survivors remain in hospital, news agencies quoted health authorities as saying on Tuesday, 16 in serious condition. Altogether 418 people have been discharged.

Despite concerns abroad at the manner and number of deaths, a poll showed 85 percent of Russians backed Putin's methods.

Police say dozens of suspects had been rounded up since the weekend in an "unprecedented drive to track down terrorists".

EXTRADITION?

Denmark said it needed more evidence to extradite Zakayev, but might do so if Moscow pledged not to use the death penalty, still on its statute book but unused since a 1996 moratorium.

"There is a possibility that we will extradite him if a number of demands are fulfilled and we get a guarantee that he will not be subject to the death penalty," Justice Minister Lene Espersen told a news conference.

Interfax news agency quoted Deputy Russian Foreign Minister Valery Loshchinin as telling the upper house of the Russian parliament that an extradition request was being drafted.

Zakayev, a deputy prime minister in Maskhadov's fugitive administration, lives in exile and says talks - which Moscow has largely ruled out - are the only way to end the war.

He warned in the Reuters interview on Sunday that Russia's nuclear facilities could be the target of future attacks by extremists unless the Kremlin began meaningful talks rather than seeking to shore up its own pro-Moscow government in Chechnya.

Russia had him in its grasp a year ago when he flew to Moscow for two hours of talks with a senior Kremlin envoy - the only face to face contact between the two sides in three years of war, which proved inconclusive.

Moscow had guaranteed his safety before he went.

Extradition looks set to pose a dilemma for the European Union, whose six-month presidency Denmark holds.

Russia sees its long conflict with separatists in Muslim Chechnya, on its southern border with Georgia, as part of the global "war on terrorism" espoused by U.S. President George W. Bush after the September 11 attacks in the United States.

Outside criticism of what human rights groups call Russia's brutal crackdown on the million-strong population there has been more muted since September 11. But European ministers have made clear pressure for a political settlement will not go away.