Thirty Afghan civilians were among nearly 100 people killed after Nato aircraft destroyed two stolen oil tankers in the north of the country earlier this month, an Afghan government investigation has concluded.

About 69 other people who died in the bombardment in Kunduz province on September 4 were Taliban fighters, an investigator told the AFP news agency on Sunday.

"Thirty civilians were martyred and nine were wounded," said Mohamadullah Bataj, one of four investigators on the team appointed by Hamid Karzai, the president.

"Sixty-nine of the Taliban were killed, armed and unarmed."

Witnesses said shortly after the attack a US aircraft had bombed the oil tankers as local villagers were trying to collect free fuel on the invitation of the Taliban fighters.

The tankers had been seized by Taliban fighters as they were travelling from Tajikistan to Kabul.

Bomb attack feared

A German commander called in the air raid fearing that the tankers would be used in an attack on their base just six kilometres away, Germany's defence minister has said.

The air raid provoked criticism from Karzai, who called it "a major error of judgement", and the European Union, which said it harmed efforts at nation-building in Afghanistan.

Speaking to the Reuters news agency on Sunday, Mohammadullah Baktash, another of the investigators, said: "We cannot say whether the air strike was neccessary or not.

"We will present our report to Karzai and it will be up to him to investigate with the Germans."

The commander of US and Nato forces in Afghanistan promised a "complete investigation" into the bombing, saying "nothing is more important than the safety and protection of the Afghan people". 

It has not yet released details of the inquiry.