Germany's defense minister said Friday he believes civilians were killed in an airstrike in Afghanistan, but insisted that the German-requested attack was necessary from a military standpoint.

Karl-Theodor zu Guttenberg said a NATO report concludes there were "procedural errors" in the Sept. 4 airstrike, but he defended the decision by a German commander to request it as "appropriate in military terms."

The commander, Col. Georg Klein, called in the NATO airstrike against two tanker trucks that had been seized by Taliban insurgents near Kunduz, fearing they could be used to attack troops.

Although the information remains contradictory, "I personally assume that there were civilian victims," Guttenberg told reporters after assessing the NATO report.

He stressed that "I regret ... every civilian victim deeply."

The report prepared by the NATO-led International Security Assistance Force remains classified, but Guttenberg was allowed to present it to members of Germany's parliament responsible for military affairs.

The top U.S. and NATO commander in Afghanistan, Gen. Stanley McChrystal, appointed a Canadian major general to lead the investigation along with officers from the U.S. Air Force and German military.

Guttenberg did not elaborate on the procedural errors that were found. However, he said the report made clear that "contradictory, confusing and at times outdated rules of engagement" had come into play in the incident.

He said that "there is considerable room for improvement at the international level, but also at a national level" and said German troops would receive appropriate instruction to prevent future misunderstandings.

Germany has more than 4,000 troops serving in northern Afghanistan and 36 have been killed so far in the mission.