MOSCOW, Aug 27: Russia said on Friday "terrorists" were behind a passenger jet crash, and a Muslim group vowing support for Chechen fighters claimed responsibility for that attack and the downing of another airliner at almost the same moment.

"According to our initial investigation, at least one of the air crashes ... came as a result of a terror attack," a spokesman for Russia's FSB intelligence service was quoted as saying.

The spokesman, Sergei Ignachenko, announced that investigators had discovered traces of hexogen, a powerful explosive with both military and civilian uses, in the wreckage of one of two planes that crashed almost simultaneously on Tuesday.

Hexogen was identified by Russian authorities in 1999 as the explosive used in a series of apartment building blasts that killed around 200 people, an attack cited by Russian President Vladimir Putin as justification for invading Chechnya.

Mr Ignachenko said no similar evidence of terrorism had yet been found in the wreckage of the other plane. There was also no official assertion of a link between the attackers and fighters in Chechnya, who have vowed to take their five-year guerilla war against Russian troops into the country's main cities.

Officials acknowledged however that they were investigating possible connections with Chechnya, where elections crucial to the Kremlin were due on Sunday. One official said the remains of a female passenger thought to be a resident of Chechnya who had been on one of the two planes were being closely examined for traces of explosive.

A spokeswoman for the ministry of emergency situations said the passenger's remains were more badly damaged and more widely scattered on the ground than any others. Investigators also said they were looking for information on another female passenger aboard the other plane, identified by Russian news agencies only by her family name, Djebirkhanova, also thought to be of Chechen origin. -AFP