The New York Times reports that the rebels have staged a major battle and brought down a Russian helicopter in Chechnya; the incident must have been quite serious since, as usual, the Kremlin chose to lie abou it.

A Russian military helicopter crashed in the mountains of Chechnya on Friday, killing at least 18 service members and marking the most lethal day for Russian authorities in the volatile North Caucasus region since late 2005.

The circumstances surrounding the crash were not fully clear. Several Russian officials told news agencies that the helicopter, an MI-8 transport, was struck by ground fire as it was ferrying troops with two other helicopters to a skirmish against rebels in the Shatoi district of southern Chechnya. But an official at the military prosecutor’s office said by telephone that an investigation had been opened and the cause of the crash had not been officially established.

All aboard were killed, Russian news agencies said.

There were indications that the battle to which the helicopter was flying was much more intense than most in recent years. The helicopter crashed in the late morning, and fighting nearby was still being reported at 4 p.m. Russian officials also said that at least three Chechen insurgents had been killed.

“The fighting continues,” a Russian official told the Interfax news agency. “We hear nonstop automatic fire and grenade explosions. We know for certain from reliable sources that three militants were eliminated.”

The Kremlin and its pro-Russian local proxies have largely had the upper hand over the remaining insurgents since late 2004, when intensive crackdowns followed the siege of a public school in Beslan by Chechen terrorists and as the pro-Kremlin Chechen formations, many of them filled with former separatists, have consolidated their hold over the republic and its government.

Two presidents of the fugitive separatist government have been killed, as has the most prominent terrorist leader, Shamil Basayev, who died in a mysterious explosion in 2006. With the separatist leadership thinned and many former insurgents lured into amnesty programs, the pace and tactical skill of the insurgents’ operations have declined markedly.

But fighting has continued sporadically, as have occasional bombings, assassinations and ambushes in Chechnya and the republics nearby.

Russia has also maintained elite troops from its federal police forces and intelligence services in the area, to continue the hunt for the remaining veteran separatists, including Doku Umarov, the movement’s latest president and military leader.

The limited details of the operation on Friday suggested that a group of insurgents had been found and that a Russian reaction force was reinforcing the area when the helicopter crashed. It was not immediately clear whether the dead were from a federal or a local unit, although the three-man helicopter crew was reported to be Russian.