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Pakistan's government has ordered the U.S. to "vacate" an air base used for suspected drone attacks, in retaliation for a NATO strike that allegedly killed two-dozen Pakistani soldiers, Fox News has confirmed.

The demand marked the latest reprisal out of Pakistan, as the U.S. and NATO allies scramble to investigate the incident. Islamabad had already ordered the country's border crossings into Afghanistan closed, blocking off NATO supply lines, after the strike. The government issued the air base demand, and pledged a "complete review" of its relationship with the U.S. and NATO, following an emergency military meeting chaired by Pakistani Prime Minister Yousuf Raza Gilani.

Pakistan's Defense Committee condemned the attack in a written statement, saying the strike was "violative of international law and had gravely dented the fundamental basis of Pakistan's cooperation" with NATO against terrorists.

"The attack on Pakistan Army border posts is totally unacceptable and warrants an effective national response," the statement said.

The government urged the U.S. to leave the Shamsi Air Base within 15 days. The U.S. is suspected of using the facility in the past to launch armed drones and observation aircraft. Pakistan made a similar demand over the summer, though officials reportedly claimed the CIA had already suspended its use of the base as a staging ground for armed drones months earlier.

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"Senior U.S. civilian and military officials have been in touch with their Pakistani counterparts from Islamabad, Kabul and Washington to express our condolences, our desire to work together to determine what took place, and our commitment to the U.S.-Pakistan partnership which advances our shared interests, including fighting terrorism in the region," the White House said in a statement Saturday.

Still, the tone of the Pakistani government's statement Saturday underscored the depth of the potential fallout after Pakistan accused NATO aircraft of firing on two army checkpoints and killing 24 soldiers. The incident early Saturday quickly exacerbated tensions between the two countries and threatened to escalate into a standoff more severe than one last year after a similar but less deadly strike.

Last year, Pakistan closed the Torkham border crossing to NATO supplies for 10 days after U.S. helicopters accidentally killed two Pakistanis. On Saturday, Pakistan went further, closing both of the country's border crossings into landlocked Afghanistan.

A short stoppage may have little effect on the war effort, but could have deadly consequences. During last year's dispute, militants took advantage of the impasse to launch attacks against stranded or rerouted trucks carrying NATO supplies.

With 24 dead in the pre-dawn incident Saturday, U.S. officials expressed regret and vowed to launch an investigation. If confirmed, it would be the deadliest friendly fire incident by NATO against Pakistani troops since the Afghan war began a decade ago.

"This incident has my highest personal attention and my commitment to thoroughly investigate it to determine the facts," said Gen. John Allen, the top commander of U.S. and NATO forces in Afghanistan.

"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."

A statement said NATO leadership remains "committed" to improving security ties with Pakistan.

In a statement by the Department of Defense and Department of State Secretaries Clinton and Panetta said they hae both been monitoring the reports of the cross-border incident and offered their condolences.

They also offered their full support of NATO's intention to investigate immediately.

The statement also said that Secretary Clinton, Gen. Dempsey and Gen. Allen each called their Pakistani counterparts and that Ambassador Munter met with Pakistani government officials in Islamabad.

The statement stressed that the U.S. diplomatic and military leaders made clear the importance of the U.S.-Pakistani partnership, "which serves the mutual interests of our people."

The leaders "pledged to remain in close contact with their Pakistani counterparts going forward as we work through this challenging time."

Brig. Gen. Carsten Jacobson, spokesman for the International Security Assistance Force, told Fox News that the air support was called in by ground forces near the border consisting of Afghan and coalition troops. Jacobson said the air support "highly likely caused the Pakistani casualties," and said it is in everybody's interest to quickly investigate the incident.

"This is an incident that obviously has implications that reach far beyond the military side, so an investigation was started straight away," he told Fox News on Saturday. He said insurgents are the only ones who would benefit from a potential conflict.

U.S. Ambassador to Pakistan Cameron Munter also acknowledged the claims that Pakistani soldiers had been killed.

"I regret the loss of life of any Pakistani servicemen, and pledge that the United States will work closely with Pakistan to investigate this incident," Munter said.

U.S.-Pakistani relations have lurched from one diplomatic standoff to the next since the U.S. raid that killed Usama bin Laden in May in a Pakistani military town.

Before retiring, outgoing Joint Chiefs Chairman Adm. Mike Mullen in September publicly accused elements of Pakistan's spy agency of helping the militant Haqqani network in attacks against the U.S. and its allies.

Most recently, the Pakistani ambassador to the U.S. resigned amid claims he engineered a memo to Washington asking for its help in reining in the military in exchange for a raft of pro-American policies. He has denied any connection to the memo, but was replaced earlier this week by democracy activist Sherry Rehman.

The latest incident triggered a new round of problems between the two countries.

Gilani told reporters he summoned Munter to protest the alleged NATO attack, according to a Foreign Ministry statement. It said the attack was a "grave infringement of Pakistan's sovereignty" and could have serious repercussions on Islamabad's cooperation with NATO. Pakistan has also lodged protests in Washington and NATO headquarters in Brussels, it said.

A Pakistani customs official told The Associated Press that he received verbal orders Saturday to stop all NATO supplies from crossing the border through Torkham in either direction. The operator of a terminal at the border where NATO trucks park before they cross confirmed the closure.

Saeed Ahmad, a spokesman for security forces at the other crossing in Chaman in southwest Pakistan, said that his crossing was also blocked following orders "from higher-ups."

The U.S., Pakistani, and Afghan militaries have long wrestled with the technical difficulties of patrolling a border that in many places is disputed or poorly marked.

Saturday's incident took place a day after a meeting between NATO's Gen. Allen and Pakistan army chief Gen. Kayani in Islamabad to discuss border operations.

The checkpoints that were attacked had been recently set up and were intended to stop Pakistani Taliban militants holed up in Afghanistan from crossing the border and staging attacks, said two local government administrators.

The Pakistani military has blamed Pakistani Taliban militants and their allies for killing dozens of security forces in such cross-border attacks since the summer. Pakistan has criticized Afghan and foreign forces for not doing enough to stop the attacks, which it says have originated from the eastern Afghan provinces of Kunar and Nuristan. The U.S. has largely pulled out of these provinces, leaving the militants in effective control of many areas along the border.

The U.S. helicopter attack that killed two Pakistani soldiers on Sept. 30 of last year took place south of Mohmand in the Kurram tribal area. A joint U.S.-Pakistan investigation found that Pakistani soldiers fired at the two U.S. helicopters prior to the attack, a move the investigation team said was likely meant to notify the aircraft of their presence after they passed into Pakistani airspace several times.

Senior U.S. diplomatic and military officials eventually apologized for the attack, saying it could have been prevented with greater coordination between the U.S. and Pakistan. Pakistan responded by reopening the border crossing.

The Associated Press contributed to this report.