Two Russian military helicopters have gone missing during a combat mission in Chechnya.
The Russian Defence Ministry said communications with the two Mi-24 gunships had been lost and a rescue team had been sent to locate them.
The helicopters have a capacity of two crew and eight passengers.
The disappearance came after Chechen separatist leader Aslan Maskhadov urged rebels to step up hostilities ahead of a constitutional referendum in the republic on 23 March.
Referendum criticised
"We will not recognise any referendum as long as the war continues," he said.
"In the coming days, the fighters will be more active than ever in order to unmask the lies of the Russian generals who say there is no war."
The referendum - hailed by Moscow as the start of a political settlement of the three-and-a-half year conflict - is to give Chechnya a new constitution that would enshrine its status as an autonomous region of the Russian Federation.
But it is opposed by rebels, and has been criticised by human rights groups who say it has been poorly prepared and will not reflect the popular will.
Thirty non-governmental organisations have petitioned President Putin to cancel it.
Helicopter incidents
A Defence Ministry spokesman told the Associated Press news agency that contact had been lost with the helicopters at 1036 local time (0736 GMT).
Two hours later, rescuers headed to the area where they were thought to have disappeared two hours later.
Rebels remain active, particularly in southern mountainous areas of the republic, and have shot down a number of Russian aircraft with shoulder-launched missiles.
In autumn last year, two helicopters were shot down within a few days of each other killing 13 people.
And last August 118 people were killed when an overloaded transport helicopter was downed by a missile outside the Chechen capital Grozny. Rebels said they carried out the attack.
The incident has been described as one of the worst disasters in Russian military history.
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