The attack in northern Kunduz province is likely to intensify Afghan public anger over such casualties, which prompted NATO commander Gen. Stanley McChrystal last June to order curbs on airstrikes where civilians are at risk.
Violence has soared across much of the country since President Barack Obama ordered 21,000 U.S. troops to Afghanistan this year, shifting the focus of the U.S.-led war on Islamic extremism from Iraq. Fifty-one U.S. troops died in Afghanistan in August, the deadliest month for American forces there since the U.S.-led invasion in late 2001.
Kunduz, a former Taliban stronghold, had been generally peaceful until insurgent attacks began rising earlier this year - perhaps an effort to control a profitable smuggling route from Tajikistan. Most of the fighting in Afghanistan this summer has been in the south and east, where U.S. and British forces operate. Germany has troops under NATO command in Kunduz and is responsible for the area.
The airstrike occurred a day after Defense Secretary Robert Gates signaled for the first time that he may be willing to send more troops after months of publicly resisting a significant increase - despite growing public opposition in the United States to the war.
A large number of civilian casualties could also stoke opposition in Germany to the Afghan mission ahead of the Sept. 27 German national elections. There are 4,050 German soldiers in Afghanistan, and polls show a majority of Germans oppose the mission.
Friday's airstrike came hours after the militants seized the tankers near the German base - possibly for a suicide attack against the base, according to German Deputy Defense Minister Thomas Kossendey.
German officials, who spoke on condition of anonymity as a matter of policy, said the strike took place 40 minutes after the commanders requested it and an unmanned surveillance aircraft determined no civilians were in the area. It was unclear whether civilians began to assemble during that time.
Taliban spokesman Zabiullah Mujahid said the hijacked trucks were headed from Tajikistan to supply NATO forces in Kabul. When the hijackers tried to drive them across the Kunduz River, the vehicles became stuck in the mud and the insurgents opened valves to release fuel and lighten the loads, he said.
Villagers swarmed the trucks to collect the fuel despite warnings that they might be hit with an airstrike, Mujahid said, claiming no Taliban fighters died in the attack.
Abdul Moman Omar Khel, member of the Kunduz provincial council and a native of the village where the airstrike happened, said about 500 people from surrounding communities swarmed the trucks after the Taliban invited them to help themselves to the fuel.
"The Taliban called to the villagers, 'Come take free fuel,"' he said. "The people are so hungry and poor."
He said five people were killed from a single family, and a man he knows named Haji Gul Bhuddin lost three sons.
Kunduz Gov. Mohammad Omar said 90 people were killed, including a local Taliban commander and four Chechen fighters.
A senior Afghan police officer, speaking on condition of anonymity because of the sensitivity of the information, said the dead included about 40 civilians.
The director of the Kunduz hospital, Humanyun Khmosh, said a dozen people, including a 10-year-old boy, were treated for severe burns.
Many of the bodies were burned beyond recognition, and villagers were burying some of those in a mass grave.
It was impossible to independently verify details because the attack occurred in an area where Taliban forces operate. Travel is risky, and the Germans refused to allow an Associated Press reporter to accompany them to the site.
Some 10 hours after the attack, German troops reached the scene at 12:30 p.m. and received fire from militants 40 minutes later, according to a Germany army statement. They returned the fire, the statement said.
Afghan President Hamid Karzai has sharply criticized the U.S.-led command for allegedly using excessive force in the war against the Taliban, alienating the civilian population. Karzai repeated those charges in last month's still-unresolved presidential election and on Friday announced he was creating a panel to investigate the attack.
"Targeting civilians is unacceptable for us," he said.
The U.S. Embassy released a statement saying it was aware of reports of civilian casualties in Kunduz and that it awaits the results of a joint investigation by NATO and the Afghan government.
"We send our condolences to those families who lost loved ones," the statement said.
Last May, U.S. warplanes struck military targets in the western Farah province, killing an estimated 60 to 65 insurgents. The U.S. said 20 to 30 civilians also died in those attacks. The Afghan government said 140 civilians were killed.
Associated Press Writer Amir Shah in Kabul contributed to this report.