ISLAMABAD  — Afghan troops and coalition forces came under fire from the direction of two Pakistan army border posts, prompting them to call in NATO airstrikes that killed 24 Pakistani soldiers, Afghan officials said Sunday. The account challenges Islamabad's claims that the attacks, which have plunged U.S.-Pakistan ties to new lows, were unprovoked.

It also pointed to a possible explanation for the incident Saturday on the Pakistan side of the border. NATO officials have complained that insurgents fire from across the poorly defined frontier, often from positions close to Pakistani soldiers, who have been accused of tolerating or supporting them.

Pakistan's political leaders and military establishment, still facing domestic criticism following the U.S. raid that killed Osama bin Laden in May, have reacted with unprecedented anger to the soldiers' deaths. They closed the country's Western border to trucks delivering supplies to coalition troops in Afghanistan, demanded the U.S. vacate a base used by American drones within 15 days and said they were reviewing all cooperation with the U.S. and NATO.

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Despite those actions, a total rupture in what both sides acknowledge is an imperfect relationship is considered unlikely. Pakistan still relies on billions of dollars in American military and civilian aid, and the U.S. needs Islamabad's help to push Afghan insurgents to engage in peace talks.

The Afghan officials, who spoke on condition of anonymity because of the sensitivity of the issue, said it's unclear who attacked the forces taking part in the joint operation before dawn Saturday, but that the soldiers were fired upon from the direction of the Pakistani border posts that were hit in the strikes.

NATO officials have previously said a joint Afghan-NATO operation was taking place close to the border and that airstrikes were called in. All airstrikes are approved at a higher command level than the troops on the ground.

The alliance has said it is conducting an investigation to determine the details. It has not commented on Pakistani claims the attacks killed 24 soldiers, but it has not questioned them.

On Sunday, Pakistan army chief Gen. Ashfaq Pervez Kayani and regional political leaders attended the funerals of the victims, including an army major and another senior officer. Soldiers took the coffins, draped with the green and white Pakistani flag, from army helicopters before praying over them.

"The attack was unprovoked and indiscriminate," said army spokesman Maj. Gen. Athar Abbas. "There was no reason for it. Map references of all our border posts have been passed to NATO a number of times."

The attack sparked popular anger in Pakistan. There were protests in several town and cities across the country, including Karachi, where around 500 Islamists rallied outside the U.S. Consulate.

A Reuters reporter at the scene said the angry crowd shouted "Down with America". One young man climbed on the wall surrounding the heavily fortified compound and attached a Pakistani flag to barbed wire.

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NATO's top official, Secretary-General Anders Fogh Rasmussen, offered his deepest condolences and said the coalition was committed to working with Pakistan to "avoid such tragedies in the future."

"We have a joint interest in the fight against cross-border terrorism and in ensuring that Afghanistan does not once again become a safe-haven for terrorists," Rasmussen said in Brussels.

The U.S.-Pakistan relationship took a major hit after the covert American raid that killed bin Laden in a Pakistani garrison town. Islamabad was outraged it wasn't told about the operation beforehand. The U.S. has been consistently frustrated by Pakistan's refusal to target militants using its territory to attack American and other NATO troops in Afghanistan.

A year ago, a U.S. helicopter attack killed two Pakistani soldiers posted on the border. A joint U.S.-Pakistan investigation found that Pakistani troops fired at the two U.S. helicopters prior to the attack, a move the probe said was likely meant to notify the aircraft of their presence after they passed into Pakistani airspace.

Islamabad closed one of the two border crossing for U.S. supplies for 10 days to protest that incident.

There was no indication of how long Islamabad could keep the border closed this time.

On Sunday, around 300 trucks carrying supplies to U.S.-led forces in Afghanistan were backed up at the Torkham border crossing in the northwest Khyber tribal area, the same crossing that was closed last year, as well as Chaman in southwestern Baluchistan province

Militants inside Pakistan periodically attack the slow-moving convoys, and took advantage last year when the trucks were waiting for days to enter Afghanistan, torching 150.

"We are worried," said driver Saeed Khan, speaking Sunday by telephone from the border terminal in Torkham. "This area is always vulnerable to attacks. Sometimes rockets are lobbed at us. Sometimes we are targeted by bombs."

Some drivers said paramilitary troops had been deployed to protect their convoys since the closures, but others were left without any additional protection. Even those who did receive troops did not feel safe.

"If there is an attack, what can five or six troops do?" said Niamatullah Khan, a fuel truck driver who was parked with 35 other vehicles at a restaurant about 125 miles (200 kilometers) from Chaman.

NATO ships nearly 50 percent of its non-lethal supplies like fuel, food and clothes to its troops in Afghanistan through Pakistan. Critical supplies like ammunition, are airlifted directly to Afghan air bases.

A NATO official closely involved with the Afghan war said there will likely be no immediate negative effect from Pakistan's decision to close its border crossings. NATO has built up a large stockpile of military and other supplies that could enable operations to continue at their current level for several months, said the official, speaking on condition of anonymity because of the sensitivity of the matter.

NATO has reduced the amount of non-lethal supplies it ships through Pakistan from a high of around 80 percent by using routes through Central Asia. The northern logistics link could be expanded to make up for the Pakistani closure, but it would leave NATO heavily dependent on Russia at a time when ties with Moscow are increasingly strained.

In addition to closing its border crossings, Pakistan gave the U.S. 15 days to vacate Shamsi Air Base in Baluchistan. Washington uses the base to service drones targeting al-Qaida and Taliban militants in Pakistan's tribal region when they cannot return to their bases inside Afghanistan because of weather conditions or mechanical difficulty, U.S. and Pakistani officials said. They spoke on condition of anonymity to discuss sensitive matters.

The drone strikes are very unpopular in Pakistan, and Pakistani military and civilian leaders say publicly that the U.S. carries them out without their permission. But privately, they allow them to go, and even help in the targeting for some of them.