Afghan troops who came under fire while operating near the Pakistan border called in the NATO airstrikes that allegedly killed 24 Pakistani soldiers at two posts along the frontier, Afghan officials said Sunday.

The officials, who spoke on condition of anonymity because of the sensitivity of the issue, said it’s unclear who attacked the Afghan troops before dawn Saturday, but that the soldiers were fired upon from the direction of the Pakistani border posts that were hit in the strikes. The border area where the soldiers were operating contains a mix of Pakistani forces and Islamist militants.

The incident has driven to new lows the United States’ already tattered alliance with Pakistan, a relationship that is vital to winding down the 10-year-old Afghan war. The Pakistan army has said the alleged NATO attack was unprovoked and has insisted there wasn’t militant activity near the border posts in the Mohmand tribal area. Outraged by the strike, Islamabad closed its border to trucks delivering supplies to coalition troops in Afghanistan and demanded the U.S. vacate a base used by American drones within 15 days.

NATO has said it is likely that its aircraft carried out the attack that caused Pakistani casualties and is conducting an investigation to determine the details. The Pakistan-Afghanistan border is disputed and not marked in many areas, adding to the difficulty.

On Sunday, Pakistan army chief Gen. Ashfaq Pervez Kayani attended the funerals of the victims, including a major, as the U.S. sought to minimize fallout from the crisis, which plunged Washington’s already troubled relationship with Islamabad to an all-time low.

The relationship took a major hit after the covert U.S. raid that killed Osama bin Laden in a Pakistani garrison town in May. Pakistan 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.

But there are forces working against a total rupture in the relationship. 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.

Tensions could rise further if militants unleash attacks against hundreds of trucks carrying supplies to U.S.-led forces in Afghanistan that were backed up at Pakistani border crossings Sunday after Islamabad closed the frontier.

Suspected militants destroyed around 150 trucks and injured drivers and police a year ago after Pakistan closed one of its Afghan border crossings to NATO supplies for about 10 days in retaliation for a U.S. helicopter attack that accidentally killed two Pakistani soldiers.

The situation could be more dire this time because Pakistan has closed both its crossings. Nearly 300 trucks carrying coalition supplies are now backed up at Torkham in the northwest Khyber tribal area and Chaman in southwestern Baluchistan province. Last year, Pakistan only closed Torkham.

“We are worried,” said driver Saeed Khan, speaking 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.”

Khan and hundreds of other drivers and their assistants barely slept Saturday night because they were worried about potential attacks, he said.

Some drivers said Pakistan had sent paramilitary troops 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? Nothing,” 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 to its troops in Afghanistan through Pakistan. The trucks are periodically targeted by suspected militants as they travel through the country, and their drivers are sometimes killed.

An 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.

Some critical supplies, including ammunition, are airlifted directly to Afghan air bases.

Pakistan eventually relented and reopened Torkham last year after the U.S. apologized. But the number of alleged casualties is much higher this time and the relationship between the two countries is much worse.

Pakistani Foreign Minister Hina Rabbani Khar told U.S. Secretary of State Hillary Rodham Clinton on Sunday that the alleged NATO attack negated all progress in improving the damaged alliance between the two countries.

She told Clinton in a phone call that the alleged NATO attack was unacceptable, showed complete disregard for human life and sparked rage within Pakistan, according to a press release issued by the Pakistani foreign minister’s office.

Islamabad also protested to the Afghan government, saying it should prevent NATO from using its territory to attack Pakistan, according to another statement from the Pakistani foreign minister’s office.

An Afghan official denounced the protest as “baseless,” saying NATO operates in Afghanistan under a U.N. mandate that is approved by Pakistan. The official spoke on condition of anonymity because of the sensitivity of the matter.

In addition to closing its border crossings, Pakistan gave the U.S. 15 days to vacate Shamsi Air Base in Baluchistan. The U.S. 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.

U.S. officials have expressed their sympathies over the incident and have promised to work closely with Pakistan as NATO carries out its investigation.

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.

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Faiez reported from Kabul. Associated Press writers Sebastian Abbot in Islamabad, Abdul Sattar in Quetta, Pakistan, Matiullah Achakzai in Chaman, Deb Riechmann in Kabul, Afghanistan, and Slobodan Lekic in Brussels contributed to this report.