A statement from the NATO-led force said Tuesday that commanders originally believed the tankers were surrounded only by Taliban insurgents, but that a subsequent review showed "civilians also were killed and injured in the strike." Previously, officials had said only that civilians may have been wounded.
The top U.S. and NATO commander in Afghanistan, Gen. Stanley McChrystal, appointed a Canadian major general to lead the investigation. A U.S. Air Force officer and a German officer are also on the investigating team, the NATO-led force announced. The make-up of the investigative team is important because the incident involved both German and U.S. forces.
Taliban militants have used attacks like the one Friday in northern Kunduz province to rally support among villagers angry at international forces.
McChrystal has said military officials could see about 120 people around the tankers when the airstrikes were launched. German officials have said they believed all were militants, but the decision to launch airstrikes appeared to run counter to directives from McChrystal to draw back from conflicts rather than risk civilian deaths.
Merkel acknowledged the possibility that civilians were harmed and that "we will not gloss over anything" when results of the investigation are clear. But she told parliament that the identities of those hit were still unclear because of contradictory reports.
"We will not accept premature judgments," she said. "I say this very clearly after what I have experienced in the last few days: I will not tolerate that from whoever it may be, at home as well as abroad."
She told parliament weeks before national elections that she regrets deeply if any civilians were harmed.
However, she also delivered a robust defense of a military mission that is unpopular at home.
The NATO announcement came the same day a Taliban car bomber attacked an international convoy near the entrance to the military airport in Kabul. The blast killed at least three civilians, Afghan officials said.
No foreign forces were killed in the attack, U.S. military spokeswoman Capt. Elizabeth Mathias said.
The Taliban claimed responsibility for the blast, the third major attack by insurgents in the capital in four weeks. The Belgian Defense Ministry said one Belgian soldier was seriously wounded and that three others were lightly wounded.
The chief of Kabul's criminal investigation department, Abdul Ghafar Sayadzada, said three Afghan civilians were killed and six wounded.
Taliban spokesman Zabiullah Mujahid said a suicide car bomber rammed into a NATO convoy and destroyed three Land Cruisers.
The early morning blast rattled windows more than a mile (1.5 kilometers) away and flames continued to shoot out from burning vehicles more than an hour later. Small blasts could be heard, likely from ammunition exploding inside the vehicles.
A witness said he saw the car ram into a line of SUVs.
"I saw three or four Land Cruisers for the foreigners just in front of the gate ... then there was a car and it hit them then blew up," said Humayun, who watched the attack from his nearby shop.
U.S. forces spokesman Lt. Col. Todd Vician confirmed an explosion south of the airport, but said he did not have details. The military airport used by U.S. and other international forces is right next to Kabul's civilian airport, but they have separate entrances.
Insurgent attacks, often deadly, occur in Kabul despite tight security and blast walls. Suicide bombers have hit government buildings and gunmen have overrun ministries.
In the run-up to the Aug. 20 presidential election, a suicide attack near the main gate of NATO headquarters killed seven people and injured scores and gunmen briefly took over a bank in the city. Also insurgents fired on the presidential palace on the same day that they unleashed a suicide car bombers on a NATO convoy.
The violence in the capital comes amid growing uncertainty over the vote. A U.N.-backed commission investigating the vote said Tuesday it had found "clear and convincing evidence of fraud" and that it was ordering a recount of questionable polling stations.
Widespread allegations of ballot-box stuffing and suspicious tallies are threatening the legitimacy of the election as the country awaits final results. More than 720 major fraud charges have been lodged with the Electoral Complaints Commission.
Meanwhile, Taliban insurgents ambushed a police convoy in the village of Dahna Ghori in Baghlan province Monday evening, Gov. Mohammad Akbar Barakzai said Tuesday, and police killed 12 Taliban in the resulting firefight.
Police suffered no casualties in the ambush, Barakzai said. But he said as the convoy was returning to Pul-e-Khumri, the provincial capital, it was hit by a bomb.
Barakzai said one policeman was killed and 17 were wounded in the explosion, two of them seriously.