U.S. Defense Secretary Robert Gates says warnings about civilian casualties from Afghan President Hamid Karzai reflect the “pain and suffering” the Afghan people have endured after 30 years of war.

Gates said Tuesday it is important for both sides to jointly investigate civilian casualties and that Mr. Karzai and the Afghan people recognize the U.S.-led NATO coalition is an ally trying to help Afghanistan see an end to the conflict.

Earlier Tuesday, Mr. Karzai warned NATO-led forces not to become an “occupying force” in Afghanistan after a spate of civilian casualties resulting from coalition airstrikes.

Mr. Karzai told reporters in Kabul he would take unspecified action if the killing of innocent civilians continues. He also stressed that the Afghan people can no longer tolerate airstrikes which target civilian houses, and said it was his last warning.

Gates said the Taliban causes about 80 percent of civilian casualties in Afghanistan, and that militants are targeting civilians with IEDs and other attacks.

In Brussels, NATO spokeswoman Oana Lungescu said Tuesday that airstrikes on buildings that might appear to be residential are necessary, but added that NATO takes Mr. Karzai's concerns seriously and will continue making every effort to prevent civilian casualties.

A spokeswoman for the International Security Forces in Afghanistan, Lieutenant Colonel Regina Winchester, said NATO forces have clear rules of engagement intended to minimize civilian casualties.

U.S. and NATO troops are in Afghanistan under a U.N. mandate that is due to expire in October. Negotiations on what the status of foreign troops will be after that date continue. President Karzai has said that he will put strict controls on the conduct of international forces in any agreement.

NATO said in a statement Wednesday that one of its service members died Tuesday in an insurgent attack in eastern Afghanistan.