Insurgents kidnapped three Afghan soldiers and another man from a bus in eastern Afghanistan and killed all four, authorities said Friday, in another challenge to the government and its security forces.

Militants stopped the bus as it was traveling in Paktia province's Ahmad Khil district and forced the four men off the vehicle Thursday, provincial deputy police chief Mohammad Zaman said. Their bullet-ridden bodies were found lying on a road later in the day.

Three of the men were off-duty soldiers who were traveling to see their families, and the fourth victim's identity was unclear, Zaman said.

The Taliban and their allies frequently kill Afghan police and soldiers to counter the plan to strengthen national security forces. They are due to take over main responsibility for fighting militants after international combat troops leave in 2014.

Elsewhere in Afghanistan, six civilians were killed when their vehicle hit a roadside bomb as they traveled in the southern province of Kandahar, police said.

The civilians were riding on a motorcycle-drawn cart when it hit the bomb Thursday in the town of Spin Boldak, provincial police chief Abdul Razaq said.

Roadside bombs are a favorite Taliban weapon to target international and Afghan forces, but the explosives often kill ordinary Afghans instead.

A U.N. report says 1,145 civilians were killed during the first half of the year, 80 percent of them by militants. Insurgent-placed homemade bombs continued to be the biggest killer of civilians, accounting for 29 percent of all noncombatant deaths during the first six months of 2012.