The officials, who spoke on condition of anonymity, 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.
Thousands gathered outside the American Consulate in Karachi on Sunday to protest against the NATO air strike.
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.
"America is attacking our borders. The government should immediately break ties with it," said Naseema Baluch, a housewife attending the Karachi demonstration.
"America wants to occupy our country but we will not let it do that.”
Many Pakistanis believe their army is fighting a war against militants that only serves Western interests and hurts their country.
"US stabs Pakistan in the back, again," said a headline in the Daily Times, reflecting fury over the attack in Pakistan, a regional power seen as critical to US efforts to stabilize neighboring Afghanistan.
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 US 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 US 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 US raid that killed Osama bin Laden in a Pakistani town in May.
Tensions could rise further if militants unleash attacks against hundreds of trucks carrying supplies to US-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 US 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 Balochistan province. Last year, Pakistan only closed Torkham.
"We are worried," said driver Saeed Khan, in Torkham. "This area is always vulnerable to attacks.
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.
Pakistani Foreign Minister Hina Rabbani Khar told US Secretary of State Hillary 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.
In addition to closing its border crossings, Pakistan gave the US 15 days to vacate Shamsi Air Base in Balochistan. The US uses the base to service drones targeting Al-Qaeda and Taleban militants in Pakistan's tribal region when they cannot return to their bases inside Afghanistan because of weather conditions or mechanical difficulty, US and Pakistani officials said.
