A U.S. warplane summoned by German troops fired on hijacked fuel trucks in Afghanistan before dawn Friday, 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," 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 U.S. 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. Media men 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 U.S. and NATO-led International Security Assistance Force (ISAF), said Afghan authorities had reported two fuel trucks hijacked. NATO aircraft spotted them on a river bank. "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," she said. ISAF spokesman Brigadier-General Eric Tremblay later said: "It would appear that many civilian casualties are being evacuated and treated in the local hospitals." A U.S. defense official said the strike was carried out by an American F-15 jet. Germany's Defense Ministry said permission to fire had been granted by a German commander on the ground. The strike took place near the border with Tajikistan, in a part of the country once seen as safe but where Taliban attacks have become increasingly frequent and fighters have asserted control of remote areas. The Taliban consider fuel shipments a strategic target because NATO forces depend on them. The Kunduz area is patrolled mainly by NATO 's 4,000-strong German contingent which is barred by Berlin from operating in combat areas further south. Germany holds a general election in three weeks and the incident will add fuel to a raging domestic debate about a war that is deeply unpopular there. Preventing civilian deaths has been one of the main themes of the new ISAF commander, U.S. Army General Stanley McChrystal, who took command in June and says the main goal of the war is to defend Afghan civilians, not hunt down insurgents. Under orders he issued in July, aircraft are not supposed to fire unless they are sure there is no chance civilians can be hurt, or they are responding to an immediate threat. The United Nations deputy envoy in Kabul, Peter Galbraith, said an investigation must answer "why an air strike was employed in circumstances where it was hard to determine with certainty that civilians were not present." A Taliban spokesman, Zabihullah Mujahid, said fighters had captured the two fuel tankers. One had become stuck in mud by a village, and the fighters went to try to tow it when residents gathered to siphon off the fuel and the crowd was struck. U.S. President Barack Obama has made stabilizing Afghanistan a foreign policy priority although public support for the war has eroded as U.S. combat deaths have risen to record levels.