Russian authorities have vowed to improve aviation security

"Both planes were destroyed as the result of terrorist acts," spokesman Lt Gen Andrei Fetisov told the Itar-Tass news agency.

Investigators say traces of explosive were found in the remains of both jets.

The two Tupolev airliners crashed within minutes of each other in southern Russia, hundreds of miles apart, killing 89 passengers and crew.

Transport Minister Igor Levitin told Russian television that the crews reported no problems, suggesting a bomb was set off without advance warning.

1. Domodedovo Airport 1735: A Sibir Airlines Tu-154 bound for Sochi departs 1815: A Volga-Aviaexpress Tu-134 leaves for Volgograd 1856: Contact lost with Tu-134 1859: Contact lost with Tu-154 2. Tula region Wreckage from Tu-134 found near the village of Buchalki soon after contact is lost 3. Rostov-on-Don 0400 (approx): Wreckage from Tu-154 found (All times in GMT) In pictures: Plane crashes Chechen terror haunts Russia

A distress signal sent from one of the planes could have been triggered by the force of the crash, he added.

Reports say investigations are focusing on two women passengers, believed to be from the restive territory of Chechnya, where presidential elections were held on Sunday.

Investigators say no-one has come forward to claim the women's bodies.

Officials had warned that Chechen separatist rebels could resort to terrorism to try to undermine Sunday's voting.

An obscure Islamist group has claimed responsibility for attacking both planes in a website statement.

The Islambouli Brigades said it would continue operations "until the killings of our Muslim brothers in Chechnya cease".