A German Army spokesman said that at least 56 of the dead were Taleban fighters killed in a Nato airstrike. The governor of Kunduz province, on the northern border with Tajikistan, said some 90 people died.

The civilians had apparently come out to take free petrol that the insurgents were offering from the hijacked trucks, one of which had apparently run aground, when the war plane struck.

The air strike occurred after Nato’s US commander had tried to shift the emphasis away from attacks that cause massive civilian casualties and alienate public opinion, and more towards protecting the population from the Taleban.

It also coincides with Gordon Brown’s policy speech today on the war, in which he will say Britain is determined to keep its forces in Afghanistan despite fading public support for the eight-year conflict.

The Prime Minister's speech was also overshadowed by the resignation last night of a defence aide, Eric Joyce, a former Army major, over the Government's handling of the conflict.

Taleban fighters hijacked the two tankers, contracted to ship fuel to German forces, late on Thursday night at a river crossing near the village of Omar Khmel, Afghan police said.

There were no German troops escorting the convoy. Several hours later, the militants started offering free fuel to the local population, who were lining up to receive their rations when the Nato bombs struck the tankers.

A US spokeswoman for the Nato-led International Security Assistance Force (ISAF) said the tankers were spotted by the pilots on a river bank and the people surrounding them were taken for Taleban fighters.

"After observing that only insurgents were in the area, the local ISAF commander ordered air strikes which destroyed the fuel trucks and killed a large number of insurgents," said Lieutenant-Commander Christine Sidenstricker.

"The strike was against insurgents. That's who we believe was killed. But we are absolutely investigating reports of civilian deaths."

There was confusion surrounding the incident, with one Kunduz police commander saying that the fireball erupted because two Taleban factions were fighting over the hijacked trucks, and insisting it was unlikely that civilians would have been out in a Taleban-controlled area at around 2 am when the explosion occurred.

In Berlin, a German army spokesman also reported a gun battle had taken place at the scene.

Locals said that civilians had been horribly torched in the explosion. "My brother was burnt when the aircraft bombed the fuel tankers. I don't know whether he is dead or alive," said Ghulam Yahya, a distraught relative who along with dozens of others had gathered outside Kunduz Central Hospital in the provincial capital.

A doctor said 13 people had been treated for burns, but that the dead were so badly cremated that there was little left to take to the morgue.

"It is very hard to collect dead bodies or remains from the blast because the fuel they were collecting was highly flammable," said Dr Mohammad Humayun Khamosh.

"We blame both the Taleban and the government," said Mohammad Sarwar, a tribal elder, who said the Taleban had offered free petrol to villagers, part of a bid to win the support of the local communities in the bitter insurgency.

A Taleban spokesman, Zabihullah Mujahid, confirmed that guerrillas had captured the tankers, and that one had bogged down in mud by a village. The Taleban fighters went to try to tow it when residents came out to collect fuel from it.