A Russian military helicopter heading to the scene of a clash with separatist rebels crashed in Chechnya, killing all 18 people aboard, emergency officials said.
There were conflicting reports about whether the craft was shot down, but the fact that it was en route to a military operation underlined that Chechnya's insurgency remains active, although significantly quieter than a few years ago.
It was the largest single-day loss of life reported by the Russian military in Chechnya in at least two years.
The Mi-8 helicopter went down Friday while flying to southern Chechnya as part of an operation against militants, an official at the regional branch of the Emergency Situations Ministry said, speaking on condition of anonymity because he was not authorized to give his name to the media.
The operation was in the Shatoi region, deep in the rugged Caucasus Mountains where small rebel bands take shelter, eluding Russian troops in the thick forests and deep ravines.
The emergency official said preliminary indications were that the helicopter - carrying 15 soldiers and a crew of three - was shot down, but a ministry duty officer in the region later called the crash an accident, saying a rotor blade had struck something during an attempt to land.
Chechen President Ramzan Kadyrov gave a third version of the crash. "The cause of the catastrophe, according to information from the scene, was a technical failure," he was quoted as saying by the RIA-Novosti news agency.
Russian forces have fought two wars against separatist rebels in Chechnya over the past dozen. The first ended in 1996 after 20 months when Russian forces withdrew after rebels fought them to a standstill.
The second began in September 1999, after Chechnya had de-facto independence for three years in which it plunged into lawlessness marked by banditry, ransom kidnappings and new violence by separatists increasingly aligned with fundamentalist Islam.
Major fighting in the second war died down years ago and Russian forces have firm control over the republic's northern flatlands. But the forces have not been able to purge the capital Grozny of rebels, and insurgents have encampments in the southern mountains.
Terrorists and soldiers continue to fight in small clashes, and the insurgents also bloody Russian forces with booby-traps and remote-detonated explosives.
The emergency official said the helicopter was one of three that was flying toward an area in southern Chechnya where a group of suspected militants had been spotted. The ITAR-Tass news agency quoted an unidentified law enforcement official as saying three militants were killed in fighting in the area.