A US warplane summoned by German troops fired on hijacked fuel trucks in Afghanistan before dawn yesterday, killing as many as 90 people in an incident that could trigger a backlash against Nato.

Nato initially said it believed the casualties were all Taliban fighters, but later acknowledged that large numbers of wounded civilians were being treated in hospitals in the area.

Villagers said their relatives were siphoning fuel from the hijacked trucks and were burned alive in a giant fireball. Patients arrived in hospitals completely covered with burns.

President Hamid Karzai's office gave a death toll of 90. It said he was deeply saddened and had sent investigators.

"No civilians must be harmed during military operations," Mr Karzai said in a statement. "Targeting civilians under no circumstances is acceptable."

The incident, which took place in the northern province of Kunduz, could reignite outrage against foreign troops two months after the new US and Nato commander in the country announced measures to stop civilian deaths he says undermine the war.

Provincial officials, who themselves could face a backlash over civilian deaths, said Taliban fighters were killed as well as civilians. Provincial Governor Mohammad Omar said he believed half of those killed were militants, while provincial police chief Abdul Razzaq Yaqubi said 55 of the 90 dead were fighters. Mohammad Sarwar, a tribal elder in the province, said Taliban fighters had hijacked the tankers and were offering fuel to a crowd of villagers when the tankers were bombed.

"We blame both the Taliban and the government," he said.

Reuters reporters saw several young men with severe burns arrive at the hospital by ambulance, where doctors said 13 people were being treated including three children.

Lieutenant-Commander Christine Sidenstricker, press officer for the US and Nato-led International Security Assistance Force said Afghan authorities had reported two fuel trucks hijacked. Nato aircraft spotted them on a river bank.