A Pakistani tribesman who was injured in a clash between Afghan forces and Taliban militants talks with relatives at a hospital in Peshawar, Pakistan, on Wednesday. Eleven Pakistani soldiers were killed in an air strike and clash between Afghan forces and Taliban militants. (Muhammad Sajjad/Associated Press)

Pakistan said a U.S. air strike killed 11 of its soldiers along the border with Afghanistan late Tuesday, and warned that the attack undermined co-operation with Washington in anti-Taliban operations.

The soldiers were killed at a border post in the Mohmand region, opposite Afghanistan's Kunar province, a Pakistani security official said.

U.S. officials said Wednesday that three of their aircraft launched about a dozen bombs into Pakistan after militants attacked coalition forces in a wooded area near a frontier checkpoint.

The strikes had "been previously co-ordinated with Pakistan," said a U.S. military statement that didn't mention Pakistani casualties. The incident was being investigated, the U.S. officials said.

The Pakistani army said the attack that killed its soldiers was "a completely unprovoked and cowardly attack" which "hit[s] at the very basis of co-operation" between Pakistan and the United States in the campaign against the Taliban and al-Qaeda forces in Afghanistan.

The two countries have been allies since shortly after the Sept.11, 2001, attacks in the United States.

In the statement issued at Bagram airbase near Kabul, U.S. forces said they used unmanned drones to follow the insurgents, then fired on them. It was not clear if the jets crossed into Pakistan's airspace.

A Predator drone unmanned aircraft takes off on a U.S. customs border patrol mission from Fort Huachuca, Ariz., last October. The U.S. has used remotely piloted drones to launch attacks on targets in Afghanistan and along its border with Pakistan. (Ross D. Franklin/Associated Press)

The U.S. has used remotely piloted drones to launch a number of attacks on targets in Afghanistan and along its rugged border with Pakistan, which refuses to allow foreign military operations on its soil.

The harsh mountain region along the Afghan-Pakistani border is difficult for journalists to access and there are conflicting reports about the fighting before the air strike, with Afghan sources accusing Pakistani troops of helping Taliban militants infiltrate from their side of the border.

Officials in Pakistan said fighting broke out Tuesday after Afghan and U.S. soldiers tried to set up a mountaintop post in a contested part of the lawless frontier and Pakistani security forces told them to withdraw.

The U.S. military statement issued Wednesday said coalition soldiers were engaged by "anti-Afghan forces" Tuesday and called in air and artillery support.

Local tribesman Damagh Khan Mohmand said Afghan soldiers had moved into the area around Speena Sooka, or White Peak, on Monday evening and were supported by foreign troops.

Khan Mohmand told journalists in Pakistan that tribesmen traded fire with the Afghan and foreign forces, and said Pakistani security forces also opened fire, although that has been disputed by the military.

Rift growing in alliance: analysts

The Associated Press quotes a Taliban spokesman, Maulvi Umar, as saying militants were resisting an incursion by Afghan and NATO troops into Pakistani territory.

State-run Pakistan Television reported Afghan and foreign forces had tried to set up a military post in the area, and fought with local Pakistani tribesmen.

Pakistan-based analysts say relations with Washington are at a low ebb, as the country's new civilian government reassesses its support for the U.S. military campaign in Afghanistan and its implications for relations with tribal and Islamist forces in Pakistan.

Afghan, NATO, and U.S. officials have separately warned that growing lawlessness in the Pakistani border region could hamper operations and increase violence in Afghanistan. Recent peace agreements between Paksitani authorities and border-based militant groups could also destabilize the Afghan situation, officials have warned.

With files from the Associated Press