NATO airstrikes that killed 24 Pakistani soldiers came just as the difficult relationship between the U.S. and Pakistani militaries was showing signs of improvement.
Only hours earlier, U.S. Marine Gen. John Allen, the coalition's top commander in Afghanistan, and Pakistan's army Chief Gen. Ashfaq Pervez Kayani concluded a meeting that sought to find common ground, a senior U.S. official told The Associated Press.
The official said the two men discussed areas of cooperation and "basically what we could do for each other."
Now, Kayani is under renewed pressure from his rank and file, intelligence sharing has stopped and Pakistan has withdrawn its offer to nudge the Afghan Taliban to the negotiation table.
On its website, the U.S. Embassy warned of possible retaliation against Americans and said some U.S. government personnel outside Islamabad were being recalled to the capital as a precaution.
The White House said Monday that President Barack Obama considers the incident a tragedy and that the administration is determined to look into the circumstances of the airstrikes.
White House spokesman Jay Carney said the president extends sympathy to the families of the dead soldiers and to the people of Pakistan. Carney said: "We take it very seriously."
A complete breakdown in the U.S.-Pakistani relationship seems unlikely, and both sides know that more is at stake than ever before.
Nevertheless, the senior U.S. official said the weekend pre-dawn raids have left the relationship "the worst it has been" _ dashing hopes of restoring ties damaged by Pakistani anger over the unilateral U.S. raid on Osama bin Laden's hide-out, and U.S. outrage that the al-Qaida leader was living not far from Pakistan's version of West Point.
Saturday's airstrikes lasted almost two hours and persisted even after Pakistani commanders pleaded with coalition forces to stop, the Pakistani army claimed Monday.
NATO described the incident as "tragic and unintended" and promised a full investigation.
Afghan officials, speaking on condition of anonymity because of the ongoing investigation, said Afghan commandos and U.S. special forces were conducting a mission on the Afghan side of the border and received incoming fire from the direction of the Pakistani posts. They responded with airstrikes.
Pakistan denies it fired first at NATO.
The poorly defined, mountainous border has been a constant source of tension between Pakistan and the United States.
NATO officials have complained that insurgents fire across the frontier into Afghanistan, often from positions close to Pakistani soldiers who have been accused of tolerating or supporting the militants. NATO and Afghan forces are not allowed to cross into Pakistan in pursuit of militants.
For its part, the Pakistani military has complained about anti-Pakistan insurgents finding safe havens in Afghanistan's Kunar and Nuristan provinces. In the area in which Saturday's attack took place, Pakistan has suffered dozens of casualties at the hands of insurgents who return across the border to Afghanistan, according to U.S. and Pakistani officials.
Allen, who was visiting at Kayani's invitation, was in Pakistan when he received word of the raid, according to the senior U.S. official. "Nine hours after that meeting started, all hell broke lose," the official said.
Before Saturday's raid, the official said, "the military-to-military relationship had stabilized and was slowly, incrementally improving. The intelligence-to-intelligence relationship had also stabilized and incrementally was improving. Now it has all stopped."
Pakistan moved quickly to retaliate. It evicted the United States from Shamsi air base in southwest Baluchistan, where some CIA drones are repaired, and shut the border to NATO supplies for Afghanistan. Islamabad also withdrew an offer to encourage Afghanistan's Taliban to the negotiation table, said a senior Pakistani security official, who spoke on condition of anonymity.
For Kayani the raid was a personal blow. Under mounting pressure from his increasingly anti-American middle-ranking officers, Kayani has tried to assuage their resentment to Pakistan's partnership with the United States and as well as the 4,000 military casualties in the fight against domestic insurgents _ more than double the deaths among U.S. and NATO troops in 10 years of war in Afghanistan.
At a National Defense University session this year, Kayani was grilled for four hours by midlevel officers who wanted to know why they were fighting this war, according to a participant who spoke on condition of anonymity because of the sensitivity of the issue.
"This is very serious for Kayani. The troops are so angry. They are supposed to be allies with the Americans, and the allies are killing them. He has to be sensitive to their feelings. He has to be careful about his own image and his own safety," said retired Lt. Gen. Talat Masood. "The trouble is Kayani cannot face his troops unless the Americans give a very strong statement that this was a genuine mistake, apologize and compensate in a very big way."
Saturday's strikes added to popular anger in Pakistan against the U.S.-led coalition presence in Afghanistan. Many in the army, parliament, general population and media already believed that the U.S. and NATO are hostile to Pakistan and that the Afghan Taliban are not the enemy.
"Whoever is a friend of America is a traitor to the land," some 400 members of Jamaat-e-Dawa, an alleged front group for the militant Lashkar-e-Taiba organization, chanted in a demonstration in Karachi, the country's biggest city.
Washington views Islamabad as key to bringing about a reconciliation to end the decade-long Afghan war and allow the United States and its NATO partners to complete a military withdrawal by 2014.
If Pakistan bows out of the peace process, it "would hugely complicate the reconciliation process," said the senior U.S. official.
He said, however, that "it wouldn't be fatal. We would have to become all the more covert to deal with Afghans on this side of the border and it is also a fact that Pakistan does not have 100 percent control over Mullah Omar and his men nor does Pakistan have the Haqqanis on that tight a leash."
He was referring to the Taliban leader, who is believed to be hiding in Pakistan, and the Haqqani network, an insurgent group that operates from the country's lawless tribal areas.
For Pakistan a break in ties risks an end to billions of dollars in U.S. military and development aid.
But an aid cutoff may affect the military less than Pakistan's civilian government, say senior Pakistani security officials. They say military aid under the Coalition Support Fund has been erratic and that Washington routinely holds up payments.
Kathy Gannon is AP Special Correspondent for Pakistan and Afghanistan. AP Islamabad Bureau Chief Chris Brummitt contributed to this report.