U.S. efforts to enlist Pakistani cooperation for peace talks with the Taliban were in limbo Sunday, as the circumstances surrounding a NATO airstrike that killed 24 Pakistani soldiers remained in dispute and Pakistan threatened to boycott an international conference on Afghanistan’s future.

The military coalition in Kabul said it was still investigating the Saturday morning incident, but a spokesman suggested a joint U.S.-Afghan operation had called in the NATO helicopters for support after coming under fire. NATO Secretary General Anders Fogh Rasumussen called it a “tragic unintended accident.” But Pakistani officials maintained that the air assault was unprovoked, sustained and continued even after the Pakistani military informed its coalition counterparts at two joint border centers that an official checkpost was under attack.

The disagreements underscored the vast fissures between the warily allied countries amid efforts to engineer a negotiated settlement to the war in Afghanistan, which U.S. officials say requires Pakistani participation. U.S. and Pakistani officials have worked to improve relations despite regular blows to their alliance, and military officials have focused on border coordination. But officials on both sides said the NATO airstrike appeared to represent a monumental failure of communication.

“This is a mess,” said a U.S. military official in Kabul, who, like other officials, spoke on condition of anonymity to discuss the sensitive situation.

In a phone call Sunday to Secretary of State Hillary Rodham Clinton, Pakistani Foreign Minister Hina Rabbani Khar said the airstrike inside Pakistani territory “negates the progress made by the two countries on improving relations.” Her office said it was now undecided about attending the Bonn conference on Afghanistan in early December. Afghan and American officials view Pakistan’s attendance — and its help with peace talks -- as important because of the influence it is believed to have over the Taliban.

The incident once again ground regular diplomatic encounters to a halt, as has happened after other recent crises, including the U.S. raid to kill Osama bin Laden in Pakistan in May. Pakistan said it is reviewing its intelligence and diplomatic relations with the United States, and officials said Sunday that various upcoming U.S.-Pakistan meetings on reconciliation and other topics had been put on hold.

“This is pretty serious,” a U.S. official said. “We should not expect this to blow over soon.”

As Pakistan buried the 24 soldiers Sunday, its two main border crossings remained closed to cargo trucks that carry nearly half of the supplies to coalition troops in Afghanistan. Interior Minister Rehman Malik said the move, enacted Saturday in response to the air assault, would be permanent. American officials view that as unlikely, but they say the blockade could last longer than the 10-day stoppage after a NATO airstrike killed two Pakistani soldiers last year.

Anger over the incident gripped Pakistan, leaving the unpopular government little incentive to soften its stance.Thousands of people protested the strike outside the U.S. consulate in the southern city of Karachi. Religious parties and banned militant outfits demonstrated in various cities, calling for retaliation against what they described as an offensive attack.

Maj. Gen. Athar Abbas, a Pakistani military spokesman, stopped short of that characterization, but he said the strike was “inexplicable.” In an interview, he said the two border posts are clearly marked and their locations are known to Afghan and coalition forces. No militant or military firing preceded the NATO assault, nor did coalition troops inform Pakistan that they were receiving fire from the Pakistani side, as is procedure, Abbas said.

Once the strike began, Abbas said, soldiers notified their commanders in the nearby city of Peshawar, who told officials at military headquarters in Rawalpindi, who then informed two trilateral border coordination centers located at the Torkham pass and the border of Pakistan’s North Waziristan region.

“But somehow it continued,” Abbas said of the firing. “Our side believes there is no possibility of confusion. The post location is not where a Taliban would take position.”

Afghan and U.S. military officials, however, say they believe Taliban fighters — many of whom are based in Pakistan’s remote tribal areas — sometimes operate alongside Pakistani troops. The United States has long alleged that insurgents are sheltered and at times aided by Pakistan, which views them as assets for influence in Afghanistan. Pakistan denies that.

In Afghanistan’s Paktika province, a border area farther south from where Saturday’s incident took place, U.S. soldiers have repeatedly come under attack by rockets fired near Pakistani border posts, sometimes within sight of these bases. Some U.S. soldiers believe Pakistani troops are complicit in these attacks, or at least do little to stop them, while others say the evidence for this is not clear. Pakistan says all military firing from its side into Afghanistan is aimed at fleeing insurgents.

Abbas said the area on the Pakistani side of the border where the airstrike occurred, Mohmand, has been “cleared completely” of militants, so none would have fired on the Afghan-U.S. operation. Leaders of anti-Taliban tribal militias that fight alongside Pakistani security forces in Mohmand supported that Sunday, and they also described a sustained NATO air assault that lasted two to three hours.

“The attack has helped militancy prevail,” said Malik Sultan Khan, who heads a militia that has been fighting the Taliban in Mohmand for three years. “We are desperate to take revenge from the U.S.”

Partlow reported from Kabul. Special correspondent Haq Nawaz Khan contributed from Peshawar, Pakistan.