Four U.S. troops died in a militant attack in eastern Afghanistan, and NATO forces acknowledged for the first time that civilians were among the dozens killed in an airstrike on two hijacked fuel trucks.

Top NATO and U.S. commander Gen. Stanley McChrystal appointed a Canadian major general to lead an investigation into Friday's strike on the fuel tankers in northern Kunduz province. An Afghan official appointed by President Hamid Karzai to examine the attack said his best estimate of the death toll was 82, including at least 45 armed militants.

Also Tuesday, McChrystal banned the sale of alcohol at the military alliance's Kabul headquarters after becoming frustrated when he had trouble getting in touch with some of his staff after the attack in Kunduz, said Capt. Elizabeth Mathias, a U.S. military spokeswoman.

Mathias said four American troops were killed in "a complex attack" in eastern Afghanistan's Kunar province but did not give details.

McClatchy Co. newspapers reported four U.S. Marines died in an ambush by insurgents. Seven Afghan troops and an interpreter also were killed in the attack and hours-long battle that followed, McClatchy's Jonathan S. Landay reported. He said the fighting took place after U.S. and Afghan forces were asked to a meeting with local elders near the village of Gangigal some six miles from the Pakistani border.

The deaths bring to 11 the number of U.S. service members killed in September. Last month, when 51 troops died, was the deadliest for American forces in Afghanistan since the U.S.-led invasion in late 2001 to oust the Taliban regime.

Fighting has intensified since President Barack Obama ordered 21,000 more troops to the country this year, and controversy after the attack in Kunduz has reopened the debate over how international forces conduct operations. Taliban militants have used reports of civilian casualties to rally support among villagers.

McChrystal appointed a Canadian major general to lead the inquiry — a delicate inquiry because the incident involved German forces ordering the airstrikes and U.S. fighter pilots carrying them out. A U.S. Air Force officer and a German officer also are on the investigating team.

German Chancellor Angela Merkel said Tuesday that her government won't accept "premature judgments" about the airstrike in Kunduz. Germany's military has been criticized for calling in the strike and for initially insisting it appeared only militants were killed.

Merkel acknowledged the possibility that civilians were harmed, but she told parliament the identities of those hit were still unclear.

Lt. Gen. Mirza Mohammad Yarmand, who Karzai named to investigate, said it was difficult to determine whether those killed were civilians or militants. He said the Taliban commander who led the hijacking, Abdur Rahman, apparently phoned supporters with tractors from surrounding villages to try to move the fuel tankers after they got stuck in a muddy riverbank. Other villagers reportedly came to collect free fuel.

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 call in airstrikes appeared to run counter to directives from McChrystal to draw back from conflicts rather than risk civilian deaths.

In Kabul, McChrystal issued the order banning alcohol sales at NATO headquarters, where more than 2,000 officers and civilians work.

The prohibition was an effort to "limit distractions" rather than comply with Islam's ban on alcohol consumption, Mathias said. The sale of alcohol to Muslims is illegal in Afghanistan.

Mathias said McChrystal had been considering the ban for some time, but she conceded he expressed frustration early Friday after he had trouble contacting some of his staff after the Kunduz airstrike.

The ban does not affect U.S. troops, who are already barred from drinking alcohol. Forty other nations participate in the NATO mission here, and some are more lenient about alcohol.