KABUL, Afghanistan -- A NATO airstrike Thursday killed at least five Afghan soldiers in one of the most devastating incidents of friendly fire in the 12-year history of the war in Afghanistan.
The troops were killed at an outpost in volatile Logar province, about 50 miles outside Kabul, according to U.S. and Afghan officials. It remains unclear how the incident occurred, and officials said it was under investigation.
There has been no issue in Afghanistan more sensitive than NATO airstrikes, which President Hamid Karzai has long protested as inhumane and ineffective. U.S. military officials have argued that they are a critical component of counterterrorism efforts. The strikes -- far less frequent than they once were -- are carried out by both unmanned and manned aircraft.
"The coalition knows the location of every Afghan outpost," said Abdul Wali, head of the Logar Provincial Council. "How can such incidents happen?"
Western officials were quick to apologize for the airstrike, which occurred before sunrise Thursday. In a statement, the U.S.-led International Security Assistance Force said the aircraft was performing reconnaissance for an Afghan operation when it "engaged suspected insurgents on a ridge."
"It was later determined that the suspected insurgents were Afghan National Army soldiers," the statement said.
In addition to the fatalities, seven soldiers were wounded, according to provincial spokesman Mohammad Darwish.
Afghan soldiers often request U.S. air support during intense firefights with the Taliban, and Afghan military officials credit that assistance with saving the lives of many troops. But the United States also maintains the ability to conduct strikes on its own, often targeting high-level insurgents.
This time, according to a Logar-based Afghan military spokesman, Mohammad Sadiq, the Afghans did not call in an airstrike.
Also Thursday, Qayum Karzai, the president's brother, announced that he would withdraw his bid for the presidency and support Zalmay Rassoul, now widely thought to be the front-runner in an election scheduled for April.
Hamid Karzai told The Washington Post this week that he had been pressuring his brother to leave the race to avoid the perception that he backed Qayum Karzai's candidacy. But that pressure has given many Afghans the impression that the president is trying to clear the way for Mr. Rassoul, his former foreign minister.