A suicide bomber killed at least 16 people in a bus packed with Russian airforce personnel heading to their base just outside the republic of Chechnya.
The woman, who was dressed in a white medical coat, waved down the bus early in the morning as it was leaving the town of Mozdok for a nearby Russian military airbase in neighbouring North Ossetia, Chechen police said.
The sources said the woman, aged around 40, shouted "Allah Akbhar" [God is Great] as she detonated a belt of explosives strapped to her body.
Sixteen people, including nine military and seven civilian workers at the base, were killed in the attack, Interfax news agency reports. Seven died on the way to hospital after being pulled out of the wreckage of the bus, police sources said.
No claim of responsibility for the attack has yet been received ,and the bomber has not been identified.
According to Chechen police sources, the suicide bomber managed to get aboard the bus, but Russian officials insist she had blown herself up after the driver kept the doors closed.
The latest attack serves as yet another reminder to President Vladimir Putin's administration that a March constitutional referendum aimed at introducing a peaceful way out of the three-and-a-half year war was not yet bearing fruit.
Mr Putin had declared an end to the war following the referendum, which cemented Chechnya's status as a part of the Russian Federation. The president has also proposed an amnesty related to the Chechen conflict, but that it has been widely criticized for not going far enough.
AFP