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 told reporters late Tuesday that 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 in the day, 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 that target civilian houses.

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

Afghanistan's intelligence chief, Lutfullah Mashal, said Wednesday that militants were increasingly using suicide bombers recruited from religious schools in neighboring Pakistan. Mashal said authorities have arrested 19 boys in the last two months.

NATO said Wednesday an Afghan-led security force captured a man it says participated in an attack last week that killed a police commander and five others in northern Takhar province. A NATO statement Wednesday said the man is part of the Islamic Movement of Uzbekistan, and was detained Monday during an operation in Balkh province.

The May 28 suicide attack in Taloqan city killed General Dawood Dawood, two German soldiers and other police officials.

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