KUNDUZ, Afghanistan — The top U.S. and NATO commander in Afghanistan is "very seriously" concerned over reports civilians might have died in an airstrike against hijacked fuel tankers, an aide said Saturday, as the alliance investigated the attack that killed up to 70 people.

A U.S. F-15E Strike Eagle jet dropped two 500-pound bombs on the two tankers early Friday after they were seized by the Taliban in Kunduz province. Local officials said the ensuing fireball killed both militants and villagers who had swarmed around the tanks to siphon off the fuel, but it was unclear how many were civilians.

The airstrike occurred despite Gen. Stanley McChrystal's orders restricting use of airpower if civilian lives are at risk. High civilian casualties in military operations have enraged Afghans and undercut support for the war against the Taliban.

An aide to McChrystal, who briefed reporters, said the general was taking reports of civilian deaths "very seriously."

McChrystal discussed the incident with Afghan President Hamid Karzai and later told senior commanders that "we need to know what we are hitting," the aide said, speaking on condition of anonymity under command policy.

McChrystal told reporters Saturday in Kabul he wanted to find out what happened in Kunduz "so that we first can prevent it from happening again — or minimize the chances that it happens again — and correct anything that we might be able to correct about it like helping the injured."

A 10-member NATO investigative team flew over the site on the Kunduz River where the U.S. jet, called in by the German military, bombed the tankers, which reportedly had become stuck trying to cross a river. German officials said the Taliban might have been planning a suicide attack on the military's nearby Kunduz base using the hijacked tankers.

The investigative team led by U.S. Rear Admiral Gregory J. Smith, NATO's director of communications in Kabul, also spoke to two wounded villagers in the Kunduz hospital, including a boy and a farmer with shrapnel wounds.

Smith said it was unclear yet how many civilians were at the site of the blast. "Unfortunately, we can't get to every village."

Mohammad Shafi, 10, who was injured in the blast and shifted to Kabul for treatment, said his father told him not to go near the stolen tankers, but he went anyway.

"While I was going to get the fuel, on the way I heard a big bang, and after that I don't know what happened," he said from his hospital bed, with bandages on his arm and leg.

A bomb blast, meanwhile, hit a German military convoy Saturday, damaging at least one vehicle and wounding four troops, none seriously. Kunduz provincial police chief, Abdullah Razaq Yaqoobi, said a suicide car bomb caused the blast, though German military officials said it was a roadside bomb.