Pakistan said Wednesday an “unprovoked and cowardly” air strike by US forces had killed 11 Pakistani soldiers on its border with Afghanistan and undermined the basis of security cooperation. The soldiers were killed at a border post in the Mohmand region, opposite Afghanistan's Kunar province, late Tuesday as US coalition forces in Afghanistan battled militants attacking from Pakistan, a Pakistani security official said. The incident came as frustration is rising in Kabul and among Western forces in Afghanistan over Pakistani efforts to negotiate pacts to end militant violence on its side of the border. NATO says such deals lead to more violence in Afghanistan. In its strongest criticism of the US military since joining the US-led campaign against terrorism, the Pakistani military condemned the killing of the 11 paramilitary soldiers, including an officer. If confirmed, it would be the most Pakistani soldiers ever killed in an attack by US forces. The attack “hit at the very basis of cooperation and sacrifice with which Pakistani soldiers are supporting the coalition in the war against terror”, the military said. “Such acts of aggression do not serve the common cause of fighting terrorism,” it said in a statement. Prime Minister Yousaf Raza Gilani also condemned the attack. “We will take a stand for sovereignty, integrity and self-respect and we will not allow our soil (to be attacked),” he told parliament. Earlier, a Pakistani security official said the soldiers were killed after militants had launched an attack into Afghanistan. “The militants launched a cross-border attack into Afghanistan ... our soldiers were killed in a counter-offensive by forces in Afghanistan,” said the official, who declined to be identified.