Relations between Washington and the new government in Islamabad have been dealt a severe blow after Pakistan angrily denounced the "unprovoked and cowardly" killing of 11 soldiers in a US air strike near the Afghan border.
The attack, which took place in the volatile tribal areas and is believed to have been carried out by a pilotless drone, is likely to sour ties between the Pakistani and American military and deepen public resentment of Pakistan's role in the so-called war on terror.
In its most vocal protest yet, Pakistan's military said the strike in Mohmand, which killed members of a paramilitary border force "had hit at the very basis of co-operation" in the fight against terrorism. It said it reserved "the right to protect our citizens and soldiers against aggression".
Yousaf Raza Gillani, the recently elected prime minister who leads a fragile coalition government, told Pakistan's parliament: "We will take a stand for sovereignty, integrity and self-respect, and we will not allow our soil [to be attacked]."
The government has been pursuing peace deals with tribal leaders and militants on the border and in the Swat valley, a move that has upset Kabul and Nato commanders in Afghanistan, who say it will lead to a surge in cross-border attacks.
While it is widely believed that previous US air strikes have killed Pakistani civilians, and possibly troops, only for responsibility to be taken by the Pakistanis themselves for political reasons, yesterday's condemnation by Islamabad broke new ground.
Precise details are still emerging. The soldiers killed, including one officer, were members of the Frontier Constabulary force manning a border post in the village of Gora Prai. The attack is reported to have taken place late on Tuesday, amid clashes between US coalition forces and militants from the Pakistani Taliban.
"Every indication we have is that this was a legitimate strike against forces that had attacked members of the coalition," Pentagon spokesman Geoff Morrell said.
In a statement issued from Afghanistan, the US-led coalition said it fired artillery and deployed pilotless drones in response to an attack. A spokesman for the Pakistani Taliban said eight of its fighters had been killed after they launched an attack.
Talat Masood, a retired general turned liberal analyst, said the soldiers' deaths would "help the militants' propaganda" and were certain to "deepen the already existing public ill will towards the United States".
"For Pakistan, it represents a loss of sovereignty and shows helplessness," he added. "Despite all the public co-operation between the two countries, it reflects a level of distrust and lack of confidence. It shows that the US does not trust Pakistanis with their intelligence, insisting that they will strike instead of letting you strike."
Mr Masood said the US was more interested in the stability of the border region than the stability of Pakistan as a whole.
On Tuesday, Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, warned that al-Qa'ida leaders in the tribal areas were plotting fresh attacks on American targets and criticised Pakistan for failing to wipe them out. His comments came as the Rand Corporation, a US-based think-tank funded by the Defence Department, claimed Pakistani agents and members of the Frontier Constabulary were helping the Taliban – a charge Pakistan denies.
Pakistan's involvement in the effort to defeat al-Qa'ida and Taliban militants within its territory, which has cost the lives of over 1,000 Pakistani soldiers, has become deeply unpopular with the public. Opponents of President Musharraf argue that it has led to the spread of terrorist attacks into major cities.