U.S. Defense Secretary Robert Gates arrived unannounced in Kabul Monday amid rising tension between Afghan President Hamid Karzai and commander of U.S. and NATO forces Gen. David Petraeus over the accidental deaths of nine boys in an airstrike.
President Barack Obama apologized for the "tragic accident," saying it undermines efforts to fight terrorism, and Petraeus also said he was "deeply sorry."
Karzai initially said U.S. apologies over the civilian death toll in airstrikes are "not enough."
"On behalf of the people of Afghanistan, I want you to stop the killings of civilians," BBC quoted Karzai as saying.
"The civilian casualties are a main cause of worsening the relationships between Afghanistan and the U.S. The people are tired of these things and apologies and condemnations are not healing any pain," he added.
U.S. Lt. Gen. David Rodriguez said the incident occurred after terrorists fired a rocket that wounded an American civilian. Rodriguez said two attack helicopters were dispatched to provide air support after troops returned fire and insurgents targeted them with another rocket.
The pilots spotted the group near the launch site and, thinking they were insurgents, fired at them, only to find out later they were boys gathering and cutting firewood. Rodriguez called it was a "terrible mistake."
Karzai's office later issued a statement announcing that Petraeus had apologized again on Sunday and promised "it will not happen again."
Meanwhile, Gates said both countries have agreed the U.S. military should remain in Afghanistan to help train Afghan forces following the end of combat operations in 2014.
"Obviously it would be a small fraction of the presence that we have today, but I think we're willing to do that," Gates told a group of U.S. troops at Bagram air field, which is headquarters for U.S. and NATO forces in eastern Afghanistan. "My sense is, they [Afghan officials] are interested in having us do that."
Without providing any details, the defense secretary added that the two governments have recently begun negotiating a security partnership.
Gates was last in Afghanistan in December to assess the war effort. This is his 13th visit as defense secretary.
AP contributed to this report.