U.S.-led forces killed Pakistani troops in an airstrike along the volatile Afghan border that Pakistan's army condemned on Wednesday as "completely unprovoked and cowardly."

The Pakistani army said the airstrike hit a post of the paramilitary Frontier Corps in the Mohmand tribal region and was a "completely unprovoked and cowardly act."

It launched a strong protest and reserved "the right to protect our citizens and soldiers against aggression," the military said in a statement. The statement said the clash "had hit at the very basis of cooperation" between the allies in the war on terror.

U.S. Ambassador Anne Patterson was summoned to Pakistan's Foreign Ministry.

"The United States regrets that actions in Mohmand agency on the night of June 10 resulted in the reported casualties among Pakistani forces who are our partners in the fight against terrorism," a U.S. Embassy statement said. It expressed condolences to the families of the dead.

In Washington, a Pentagon official said there was an airstrike during an incursion by insurgents into Afghanistan from Pakistan. The aircraft launched an airstrike under a policy that allows coalition forces to cross into Pakistan if they are in hot pursuit of a target, said the official, who spoke on condition of anonymity because he was not authorized to speak on the record.

But it was not clear if the aircraft actually crossed into Pakistan.

In a statement issued from Afghanistan, the coalition said it had retaliated after its forces came under small arms and rocket-propelled grenade fire about 200 yards inside Afghanistan's eastern Kunar province during an operation that had been "previously coordinated" with Pakistan. The coalition fired artillery, and then using drones to locate more "anti-Afghan forces," launched airstrikes "until the threat was eliminated."

The Taliban said eight of its fighters died in the skirmish.

The coalition said that it had informed the Pakistan army that it was being attacked from a wooded area near the Pakistani checkpoint at Gorparai — where the Pakistani Frontier Corps troops were killed.

Pakistan army spokesman Maj. Gen. Athar Abbas denied the insurgents attacked from Pakistan or that there had been any attack launched from the Gorparai post. He also denied the coalition had given prior notice of its operation in the area.

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

The Afghan forces "were on their way back and they were attacked by insurgents in their own territory," he said, adding that Afghans had called in coalition airstrikes which hit the Pakistani Frontier Corps troops across the border.

In Islamabad, Prime Minister Yousuf Raza Gilani said Pakistan "vehemently condemned" the airstrike.

"We will take a stand for the sovereignty, dignity and self-respect of this country," he told Parliament.

The lawless and remote mountain region is difficult for reporters to access and there were conflicting reports over the sequence of events and how many died in the fighting. The region is believed to be used by pro-Taliban militants as a launch pad for attacks into Afghanistan.

That infiltration is a constant source of tension in the counterterrorism alliance. Pakistan has deployed tens of thousands of troops to police its tribal regions, but Western and Afghan officials say that has not deterred militants. Afghanistan often accuses Pakistan of abetting the Taliban, whose hardline regime it supported until its ouster in 2001.

Local tribesman Damagh Khan Mohmand said Afghan forces moved into the area around Speena Sooka, or White Peak, on Monday evening and were supported by foreign troops. There was no confirmation of that from the U.S.-led coalition or NATO security force in Afghanistan.

Khan Mohmand said tribesmen traded fire with the Afghan and foreign forces, and said Pakistani security forces also opened fire — although the military disputed that.

Khan Mohmand said he saw drones and that two aircraft had bombed several locations.

Maulvi Umar, a spokesman for an umbrella group of Pakistani Taliban, said militants had resisted an incursion into Pakistan.

He said between 60 and 100 of its fighters attacked NATO and Afghan army troops who had set up bunkers and tents on Pakistani soil. He claimed up to 40 Afghan troops were killed, several captured and that a NATO helicopter was shot down. Eight Taliban troops also died in the fighting, he said.

None of his claims could be independently confirmed.

State-run Pakistan Television said 18 people died in the fighting, including 10 troops and eight civilians.

Officials in Afghanistan all declined comment. The Afghan Ministry of Defense said it had no information on the incident.

On Wednesday, two helicopters brought the bodies of 11 troops killed 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 because he is not allowed to comment to the media.

North West Frontier Province Gov. Owais Ahmed Ghani told reporters later at a funeral ceremony for the troops that such an attack "can compel us to review our policy (in the war on terror)."

Anti-U.S. sentiment is already running high in Pakistan, where the newly elected civilian rulers are seeking to broker peace with militants to curb an explosion in extremist violence.

Western officials are concerned that peace deals could give more space for Taliban and al-Qaida militants to operate.