A bus and truck collided and burst into flames in eastern Afghanistan on Friday, killing at least 51 people.

The collision occurred in Ghazni province on a highway that links the Afghan capital of Kabul with Kandahar, the largest city in the south. At least 51 of the 56 passengers on the bus were killed, said Gen. Zarawar Zahid, the provincial police chief.

"It is very sad," he said of the tragic accident. "When you see the bodies, some of them are not recognizable."

He said Afghan police and soldiers were still removing those killed from the wreckage along Highway 1 in Ab Band district. The fate of the two drivers was not immediately known.

Zahid said the cause of the crash has not yet been determined but he ruled out the possibility of an attack.

Meanwhile, a spokesman for Paktika province said Friday that six civilians died after the tractor they were riding in hit a roadside mine on Thursday. Mokhlis Afghan said the civilians were returning home from collecting firewood in the mountains when they struck the bomb in Khoshamand district.

Separately, the U.S.-led military coalition said a NATO service member was killed Thursday during an insurgent attack in eastern Afghanistan. No other details were released about the death.