ROSTOV-ON-DON, Russia - A Russian military helicopter heading to a battle with separatist rebels crashed in Chechnya yesterday, killing all 18 people on board, emergency officials said.

There were conflicting reports about whether the craft was shot down or a rotor accidentally hit something as it was landing.

The crash underlined the persistence of Chechnya's insurgency years after the most recent major fighting. The Mi-8 helicopter - carrying 15 soldiers and a crew of three - was one of three flying toward an area in southern Chechnya where a group of suspected insurgents had been spotted, an official at the regional branch of the Emergency Situations Ministry said. He spoke on condition of anonymity because he was not authorized to give his name to the media.

It was the largest single-day loss of life reported by the Russian military in Chechnya in at least two years.

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 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, whose pro-Kremlin administration has increased control over the region but been unable to wipe out the rebels, 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 since 1994.