At least 17 people were killed when their troop-carrying helicopter was shot down in Russia's volatile province of Chechnya on Friday, Russian news agencies and the military said.
The incident appeared to be the deadliest single attack for over a year by Chechnya's insurgents. They have fought a 12-year war against Moscow's rule but in the past few years most of their leaders have been killed and their attacks curtailed.
A spokesman for the military prosecutor's office said: "According to our information, a helicopter came down as a result of which all those on board were killed, that was four crew and 13 troops."
Russian news agencies, quoting military sources, said 18 people on board the helicopter had died.
They said the Mi-8 transport helicopter was shot down by Chechen rebels. The military has made no official comment on what caused the helicopter to go down.
The downed helicopter was one of three carrying troops to take part in an operation against the insurgents near the town of Shatoi in southern Chechnya, RIA news agency quoted a source in the Russian regional military headquarters as saying.
Gunfight raging
The agency also said a gunfight was raging at the scene of the crash between insurgents and Russian forces.
Interfax news agency quoted a source in Chechnya's pro-Moscow security forces as saying a senior insurgency leader could be among the rebels now fighting Russian troops at Shatoi.
The air force, in a statement, confirmed the aircraft had come down but said nothing about the cause.
"At 11:34 (0734 GMT) while carrying out a mission to transport troops to the south of Shatoi there was an emergency situation which involved a Mi-8 helicopter coming down."
Russia has been fighting in Chechnya since 1994, when then President Boris Yeltsin, who died this week, sent in troops to put down a separatist rebellion.
President Vladimir Putin vowed to squash the rebels when he took office seven years ago. Moscow's forces, working with local allies, have killed the main separatist leaders and driven the rebels into mountain hideouts.
The biggest loss of life sustained by Russian and pro-Moscow forces last year was when seven troops were killed during an operation near Shatoi.
Chechnya's pro-Moscow authorities said dozens of rebel fighters gave themselves up under an amnesty late last year.
But the separatists insisted they had not been finished off and pledged to keep attacking Russian forces.