A NATO airstrike in northern Afghanistan on Friday killed around 90 people, most of them Taliban insurgents, a spokesman for the provincial government said.

The NATO-led International Security Assistance Force (ISAF) said it was investigating reports of civilian casualties in an airstrike that destroyed two fuel tankers hijacked by the Taliban in northern Kunduz province.

"Some 90 people were killed in this incident and most of them are Taliban. It was an ISAF force airstrike," Mahbubullah Sayedi, a spokesman for the Kunduz provincial government, told AFP.

"A small number of the casualties are local civilians, including a few children who had come to take free fuel," he added, declining to give any further details.

The German army, whose soldiers are based in Kunduz under NATO command, said the airstrike killed 56 Taliban militants after they attacked an alliance convoy.

"There were no civilian casualties. There were no German casualties," an army statement said.

Civilian casualties during Western military operations in Afghanistan are hugely sensitive and a major source of tension with the Afghan government.