A woman suicide bomber ambushed a bus carrying Russian air force pilots near rebel Chechnya on Thursday, blowing it up and killing herself and at least 17 others, government officials said.

The attack was the third in three weeks by women suicide guerrillas fighting for Chechen independence and came on the eve of a Russian parliament vote on a partial amnesty for rebel fighters designed to improve prospects for a Kremlin peace plan.

The attack occurred in Russia's North Ossetia region, bordering Chechnya, after the bus carrying the pilots and a group of civilians attached to the air force stopped near a railway crossing on the outskirts of Mozdok.

"A terrorist-suicide woman blew it up," defense ministry spokesman Nikolai Deryabin said.

"At the present moment we have 18 people killed, including the suicide woman, and nine wounded. The condition of two of those in hospital is very serious," emergencies ministry official Vladimir Ivanov, reached by telephone, told Reuters

from Mozdok.

Officials said the woman tried to board the bus, which stopped to take on workers, and then detonated the bomb when she was unable to do so.

In Moscow, Russia's prosecutor general told President Vladimir Putin, who vowed to crush the Chechen rebellion when he took office in 2000, that 16 people had been killed. About 12 others were wounded.

Efforts would be launched to find out where and how suicide fighters -- a relatively new phenomenon in Chechnya -- were being trained, Vladimir Ustinov said in a televised encounter with Putin. "We will locate their training centers and trace their funding sources," he said.

Eight servicemen were among the dead, including four air force officers, officials said.

Putin, who will seek re-election early next year, has been pushing a peace plan for Chechnya following a Kremlin-organized referendum in March that locked the region into Russia.

North Ossetia is the springboard for Russian military operations in

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Chechnya and has escaped relatively unscathed from violence since the conflict erupted in 1994.

Russian officials assumed the attack was the work of Chechen rebels.

On May 12 a woman was part of a group that drove a truck packed with explosives into a government complex in Znamenskoye in northern Chechnya, killing 59 people. Two days later a woman blew herself up at a Muslim festival in another part of Chechnya, killing at least 16 people.

Chechen separatist warlord Shamil Basayev has claimed responsibility for the two attacks in May and threatened to launch "a whirlwind" of violence in the future.

Last March the Kremlin, as part of its peace plan, staged a constitutional referendum that anchored the territory in Russia and has set dates for parliamentary and presidential elections.

As part of the plan the State Duma, Russia's lower house of parliament, was to take a final vote on Friday to bring a partial amnesty into force.

Chechen rebels and human rights groups have ridiculed the limitations of the amnesty offer, saying it excludes hard-core fighters while benefiting many Russian servicemen suspected of abuses in the region.