Pakistan's army reacted angrily, calling the "unprovoked" raid on two Pakistani border posts an "irresponsible act." The army said NATO helicopters and fighter aircraft, under the cover of darkness, had bombed the posts in Mohmand tribal region, a lawless border area that which abuts Afghanistan's eastern Kunar province.
"Pakistan's sovereignty was attacked early this morning," said Pakistani Prime Minister Yousuf Raza Gilani. "This is our Pakistan and we have to defend it."
In retaliation, Pakistan's security forces began to turn back scores of Pakistani-owned trucks that carry NATO supplies into Afghanistan.
The U.S.-led coalition in Afghanistan said it was looking into the latest Pakistani claim. The incident took place hours after Gen. John R. Allen, the coalition commander, met with government officials and army officers in Pakistan to discuss border issues.
"This incident has my highest personal attention and my commitment to thoroughly investigate it to determine the facts," said Gen. Allen. "My most sincere and personal heartfelt condolences go out to the families and loved ones of any members of Pakistan Security Forces who may have been killed or injured."
The U.S. claims Pakistan, while waging a three-year-old war against militants in the tribal regions, has continued to shelter some factions of the Taliban as a way to maintain influence inside Afghanistan after most international troops leave in 2014. U.S. military officials say NATO troops have repeatedly come under attack from Taliban forces based over the border and have urged Pakistan to do more about militants in its tribal regions.
But President Barack Obama's administration is also nudging Pakistan to use its influence over the Taliban, which Pakistan's military helped create in the 1990s, to bring them to the negotiating table to end the 10-year war in Afghanistan. Secretary of State Hillary Clinton brought this dual message to Pakistan during a visit to Islamabad, the capital, in October, asking for stepped-up military action on Pakistan's side of the border but promising to keep Pakistan fully abreast of developments in Afghanistan, including nascent peace talks.
Pakistan's army chief, Gen. Ashfaq Parvez Kayani, faced with growing anti-U.S. sentiment, deepened by incidents like the one on Saturday, faces limited room to accede to any U.S. demands at the moment, said Talat Masood, a retired general and defense analyst.
"Those who have been more moderate, even those people are asking is it worth having a relationship with the U.S.," Mr. Masood said. "It will be very difficult for Gen. Kayani to defend the alliance."
Mr. Masood said he had taped a television chat show Saturday after the attack on the border posts during which he was the only participant arguing the U.S. wouldn't have targeted Pakistani soldiers in Mohmand as a deliberate act of aggression.
Western diplomats in Kabul, the capital of Afghanistan, said Saturday the raid is likely to hurt efforts to get Pakistan to play a significant role in forging peace talks, which are expected to take center stage at an international conference on Afghanistan to be held in Bonn, Germany, next month.
U.S. and Afghan officials say Pakistan continues to hold sway over the Taliban group controlled by Mullah Mohammed Omar, believed to be based in the western Pakistani city of Quetta, and the Haqqani faction, which shelters in North Waziristan, a Pakistani tribal region on the Afghan border. Pakistan denies this and blames the U.S.'s war in Afghanistan for sparking a war on its side of the border in which more than 3,000 Pakistani soldiers have died.
Few observers, though, expect a complete breakdown in relations. Pakistan might close its borders for a few days, temporarily hurting NATO's supply chain, but the country will continue to rely on billions of dollars in military and civilian aid from the U.S. Washington, likewise, needs Pakistan to keep up pressure on Taliban militants in the tribal region, and as a supply route, as it tries to work out an exit strategy from Afghanistan.
"This is a need-based relationship. It will have its temporary hiccup, probably in the form of the suspension of NATO cargo," said Imtiaz Gul, director of the Center for Research and Security Studies, an Islamabad-based think-tank.
In September 2010, a NATO helicopter attack on a Pakistani border post in the tribal regions killed two soldiers. Pakistan closed traffic for NATO convoys for a few days but later reopened the route. The U.S., wary of its part-time ally, begun moving more supplies for Afghanistan through Central Asia. The Pakistan land route, from the port city of Karachi across country to two major borders with Afghanistan, still accounts for roughly half of NATO supplies coming in to Afghanistan.
Since that incident, which blew over, the U.S.-Pakistan relationship has deteriorated. Pakistan' army was embarrassed and angered by the covert raid by U.S. Navy SEALs in May that killed Osama bin Laden in a Pakistani army garrison town. That came after a Central Intelligence Agency contractor shot dead two armed men in Lahore in January and was briefly jailed.
Just last week, Pakistan's ambassador to the U.S. was forced to resign amid allegations he had sought Washington's help to reduce the power of Pakistan's army, which plays a large role in domestic politics.
Pakistan's army, in response to growing anti-U.S. feeling, has began to more forcibly challenge the U.S. in public, including attacking Washington's policy of stepped-up unmanned drone strikes against Taliban targets in the tribal regions.