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 damaged 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.

Pakistan relies on billions in U.S. aid

On Sunday, Pakistan army chief Gen. Ashfaq Pervez Kayani attended the funerals of the victims as the U.S. sought to minimize fallout from the crisis, which plunged Washington's already troubled relationship with Islamabad to a new 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.

Afghanistan-bound containers carrying supplies for NATO forces remain parked after authorities closed the Chaman border crossing in Pakistan. Arshad Butt/Associated Press

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.

"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."

Some drivers said Pakistan had sent paramilitary troops to protect their convoys since the closures, but others were left without any additional protection.

Critical supplies airlifted

NATO ships nearly 50 per cent 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.

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 UN mandate that is approved by Pakistan. The official spoke on condition of anonymity because of the sensitivity of the matter.

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."