Kunduz, Afghanistan ? NATO launched an investigation Friday into a predawn airstrike in which an American fighter jet bombed two hijacked fuel trucks in northern Afghanistan, killing scores of people and prompting accusations that many of the dead were civilians.

Estimates of the number of dead varied widely. The governor of Kunduz province, where the attack occurred, said 72 people were killed when the trucks exploded in a huge fireball, while the German military said the death toll was about 50. German forces under NATO command have responsibility for the area and had called in the airstrike out of concern that the Taliban intended to use the trucks in suicide bombings, German officials said.

A senior U.S. defense official said 56 people were counted around and on top of the vehicles in video taken immediately before the blast. “Nobody that was close … lived,” he said. “But it’s also not clear whether the individuals who are around the vehicles there were actually insurgents or not.”

A Belgian soldier who visited the village nearest the site of the attack, Staff Sgt. Filip Bergeman, said residents told him that Taliban fighters forced several villagers to help remove fuel from the trucks after the vehicles got stuck in the Kunduz River. Bergeman said he was told that 14 of the villagers were killed.

The airstrike occurred a day after Defense Secretary Robert Gates indicated that he is open to increasing U.S. troop strength in Afghanistan despite earlier concerns about an outsize American “footprint” in the country. Gates said his view had been altered by an assessment submitted this week to President Obama by Gen. Stanley McChrystal, the top U.S. commander in Afghanistan, which stressed the importance of troops’ interactions with civilians and efforts to reduce civilian casualties.

President Hamid Karzai also ordered an investigation into the strike, saying that “targeting civilians is unacceptable for us.” Civilian deaths as a result of NATO military operations have fueled intense anger among many Afghans, including Karzai, and have sapped public support for the multinational mission to combat the Islamist Taliban movement, which has gained ground this year against a government widely viewed as corrupt and incompetent.

In a taped message Friday night, McChrystal appeared to acknowledge the likelihood of civilian casualties in the airstrike, saying, “I take this possible loss of life or injury to innocent Afghans very seriously.”

Translations of the message into the two main Afghan languages, Pashto and Dari, were distributed to the Afghan media. Addressing “the great people of Afghanistan,” McChrystal said the International Security Assistance Force had “launched an attack against what we believed to be a Taliban target in Kunduz. As commander, nothing is more important (to me) than the safety and protection of the Afghan people.”