NATO officials said Afghan and US troops operating inside Afghanistan early Saturday had been fired on from the Pakistani side of the border and had requested close air support to help defend themselves. What happened next is still under investigation, officials said.
But Pakistan's chief military spokesman said he did not believe that there had been any fire directed at the Americans from Pakistan and said he did not believe the attack could have been inadvertent.
Major Gen. Athar Abbas said the military outpost on a mountain top at Salala in the Mohmand part of Pakistan near the Afghan border was well marked on maps that both Pakistan and NATO have and that the US air assault lasted for more than an hour.
"I cannot rule out the possibility that this was a deliberate attack by ISAF," Abbas said, referring to NATO's International Security Assistance Force by its acronym. "If ISAF was receiving fire, then they must tell us what their losses were."
No NATO casualties have been acknowledged in Saturday's clash. A military official in Washington identified the NATO forces involved as American.
The Saturday incident was by far the worst to date between the two supposed allies along the rugged Afghan-Pakistani border and sent US-Pakistani relations to their lowest point since the May raid on Osama bin Laden's hideout, when US troops entered Pakistan without notifying Pakistani officials and killed the al Qaida leader in the Pakistani city of Abbottabod. US officials believe bin Laden had lived for years in Abbottabod, the site of the Pakistan's premier military academy.
In Pakistan, a nation already bursting with anti-Americanism, public opinion was further riled by images of the funeral of the soldiers killed, which filled television screens Sunday. The army chief, Gen. Ashfaq Kayani, considered Pakistan's most powerful man, attended the funeral services at a military base in Peshawar as did the top civilian officials from the north west.
Television images showed 24 coffins laid out on a lawn, each wrapped in a Pakistan flag. Each coffin was carried away by an honor guard to be buried in the soldiers' home towns and villages.
In a statement, NATO Secretary General Anders Fogh Rasmussen called Saturday's events a "tragic unintended incident." But NATO provided no details on what took place, and US Army Col. Greg Julian, a NATO spokesman, said officials are still trying to determine what happened after the joint US and Afghanistan army units took fire from inside Pakistan.
US officials in Washington, after a flurry of statements offering condolences Saturday, made no pronouncements Sunday.
The Pakistan positions hit are located about 300 yards inside Pakistan, Abbas said. The attack lasted for more than an hour, Abbas said, during which ISAF troops made "no attempt" to contact the Pakistani side using the established border co-ordination system. He said that the map references to the Pakistani positions had been previously passed to ISAF.
"This was a totally unprovoked attack. There are no safe havens or hideouts left there (for militants) in Mohmand," Abbas said. "This was a visible, well-made post, on top of ridges, made of concrete. Militants don't operate from mountain tops, from concrete structures."
Taliban fighters often use Pakistan's tribal area as a sanctuary, from which to launch artillery or rockets, or as a place to retreat under fire from NATO.
In the past, much confusion has been caused by insurgents firing into Afghanistan from positions close to Pakistani check points, making it appear to NATO and Afghan troops that they are coming under attack from the Pakistani positions.
In an incident last year, Pakistani soldiers shot into the air to warn NATO helicopters that they had crossed the border, but that was mistaken by the aircraft crew for incoming fire.
ISAF commanders, however, suspect that Pakistani forces look the other way when their territory is being used by Afghan insurgents, or even actively collude in the attacks. Pakistan angrily denies the charge.
The Pakistani military has repeatedly proclaimed that it has cleared Mohmand of Taliban and other extremists, only to have to launch new operations. Local anti-Taliban militias operate in Mohmand to try to keep the extremists away.
Earlier this month, Maj. Gen. Daniel Allyn, who commands troops along Afghanistan's eastern border with Pakistan, told reporters at the Pentagon that there are on average three to four cross border attacks a week. Most are blamed on the Taliban-allied Haqqani network, which US officials have claimed is allowed to operate in Pakistan.
Pakistan announced Saturday that it would "review" all military, intelligence and diplomatic co-operation with the United States and ISAF forces in response to the incident.
Pakistan also closed its border with Afghanistan to trucks carrying supplies for ISAF and announced that American forces would be expelled from Shamsi, a remote air base in Pakistan secretly turned over to US forces after the 9/11 attacks and used as a launching point for the US's highly controversial drone program.
On Sunday, Pakistan raised the possibility that it would also boycott next month's international Bonn conference, which is supposed to chalk out Afghanistan's future.
Tehmina Janjua, the spokesperson for Ministry of Foreign Affairs, said the Bonn issue is "is being examined and no decision has yet been taken in this regard." The conference is scheduled to begin Dec. 5. Pakistan's cooperation is considered vital for stabilizing Afghanistan and bringing the Taliban into negotiations.
(McClatchy special correspondent Shah reported from Karachi, Youssef, from Washington.)