Russian officials say at least 15 people have been killed and 12 injured in a suicide bomb attack on a bus in the North Caucasus.
The bus was carrying air force personnel and civilians
The explosion happened near the city of Mozdok in North Ossetia, which neighbours the breakaway republic of Chechnya.
The bus was carrying Russian air force personnel and civilians.
The bomber, a woman, approached the bus as it was about to pull away from a stop and blew herself up after failing to open the doors, Russian Deputy Prosecutor-General Sergey Fridinsky said.
Some reports say she threw herself under the bus.
Several passengers were killed instantly and others died on their way to hospital.
Mozdok is the headquarters of Russian forces which have been fighting rebels in Chechnya for most of the past decade.
The bus was carrying up to 30 air force personnel from Mozdok to the main Russian air base at Prokhladny, six kilometres (four miles) away.
Some of the passengers may have been pilots flying missions over Chechnya.
There were no suicide bombers in Chechnya before, it has
come to us from other countries
With large-scale operations in Chechnya now rare, the military are still carrying bombing raids against targets in the mountains, trying to destroy rebel bases there.
There are strict rules prohibiting military buses from picking up passengers following a spate of attacks on military vehicles in the region.
Russian officials said the woman who blew herself up was disguised as a medical worker.
The BBC's Nikolai Gorshkov in Moscow the increase in suicide bombings in recent months is deeply worrying for Russian officials.
The bus was on its way to a nearby air base
He says rebel field commanders in Chechnya appear not to be in control of the people carrying out the attacks.
But Prosecutor-General Vladimir Ustinov pledged to locate training centres and funding sources for the bombers.
"There were no suicide bombers in Chechnya before. It has
come to us from other countries," he was quoted as saying in a meeting with President Vladimir Putin.
The latest bombing comes a day after the Russian parliament approved an amnesty for Chechen rebels to kick-start a political settlement to the conflict.
Critics say Moscow should engage the rebel leaders in peace talks, but such suggestions have been dismissed by the Kremlin.