The U.S. military replied in a statement that it had coordinated the artillery and airstrike with Pakistan, but that it was investigating further, according to Reuters.
The incident, which took place late Tuesday, followed a reported battle between Afghan forces and Taliban militants in the same area. The Taliban said eight of its fighters died in the clash, reported the Associated Press.
The Pakistani army said the coalition airstrike hit a post in the paramilitary Frontier Corps and was a "completely unprovoked and cowardly act."
A Pentagon official said Wednesday that there was an airstrike Tuesday night during an incursion by insurgents into Afghanistan from Pakistan, and coalition forces responded. The United States has about 34,000 soldiers in Afghanistan, part of an international presence totaling 60,000. The official, who spoke on condition of anonymity, said the details were still sketchy, according to the AP.
Two helicopters brought the bodies of 11 troops and another 13 soldiers wounded in the fighting to Peshawar, the main city in northwestern Pakistan, a military intelligence official said on condition of anonymity, the AP reported.
The lawless, mountainous region along the Afghan-Pakistan border is a constant source of tension in the counterterrorism effort. The region is believed to be used by pro-Taliban militants to launch attacks into Afghanistan.
In March, three bombs, apparently dropped by an American aircraft, killed nine people and wounded nine others in the tribal area, and in January, one of Osama bin Laden's top lieutenants, Abu Laith al-Libi, was killed by two Hellfire missiles launched from a Predator surveillance aircraft.
Pakistan has sent tens of thousands of troops to its tribal regions, but Western and Afghan officials say that has not tempered the attacks. Afghanistan officials often accuses Pakistan of aiding the Taliban.
The Taliban's hardline regime was ousted from Afghanistan in 2001.