KABUL, Afghanistan — Taliban militants seized a civilian bus in volatile southern Afghanistan and executed as many as 30 passengers, beheading some of them, officials said Sunday.

The Thursday attack took place in Kandahar province, which was the home base of the militant Islamic movement before it was toppled by a U.S.-led invasion in late 2001.

Violence in Afghanistan this year has hit its highest levels since the conflict began. Many of those killed have been combatants, but civilian deaths in 2008 so far have been estimated by the United Nations at more than 1,300.

Afghan authorities said the passengers — many of them women and children — were traveling in a two-bus convoy, a measure intended to bolster their safety. Militants manning a makeshift checkpoint fired on the first bus, which accelerated away from them. They were able to halt the second, with about 50 people on board.

Between 24 and 30 passengers were killed execution-style at the scene, according to Police Chief Matiullah Khan of Kandahar province.

Western news agencies quoted a Taliban spokesman as saying that 27 people were dragged from the captured bus and shot, and that they were Afghan army soldiers. The army denied that, saying regulations prohibit soldiers from traveling in the area by civilian transport.

Some of those aboard the seized bus were reported to have been freed, but a precise count was impossible to obtain.

A spokesman for the Afghan defense ministry, Gen. Mohammed Zahir Azimi, put the death toll at 31. He said six decapitated bodies were recovered elsewhere in the district, together with about 25 bodies found at the scene.

Elsewhere in the south, U.S.-led troops killed three militants in weekend clashes in Helmand, military and Afghan authorities said.