NATO helicopters and fighter jets attacked two military outposts in northwest Pakistan on Saturday, killing as many as 28 troops, prompting strongest condemnation from Islamabad.

Pakistan retaliated by shutting down vital NATO supply routes into Afghanistan, used for sending in almost half of the alliance's non-lethal materiel, Reuters reported.

The attack is the worst single incident of its kind since Pakistan allied itself with Washington in the days immediately following the Sept. 11, 2001 attacks on U.S. targets.

The cross-border attack outraged the Pakistani government and military.

"This is an attack on Pakistan's sovereignty," said Prime Minister Yusuf Raza Gilani. "We will not let any harm come to Pakistan's sovereignty and solidarity."

The Foreign Office said it would take up the matter "in the strongest terms" with NATO and the United States.

The powerful Chief of Army Staff, General Ashfaq Pervez Kayani, said in a statement issued by the Pakistani military that "all necessary steps be under taken for an effective response to this irresponsible act.

"A strong protest has been launched with NATO/ISAF in which it has been demanded that strong and urgent action be taken against those responsible for this aggression."

The commander of NATO-led forces in Afghanistan, General John R. Allen, said he had offered his condolences to the family of any Pakistani soldiers who "may have been killed or injured" during an "incident" on the border.

The U.S. embassy in Islamabad also offered condolences.

"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," ambassador Cameron Munter said in a statement.

Two military officials said that up to 28 troops had been killed and 11 wounded in the attack on the outposts, about 2.5 km (1.5 miles) from the Afghan border. The Pakistani military said 24 troops were killed and 13 wounded.

It remains unclear what exactly happened, but the attack took place around 2 a.m. (2100 GMT) in the Baizai area of Mohmand, where Pakistani troops are fighting Taliban militants.

"Pakistani troops effectively responded immediately in self-defense to NATO/ISAF's aggression with all available weapons," the Pakistani military statement said.

About 40 Pakistani army troops were stationed at the outposts, military sources said. Two officers were reported among the dead.

"The latest attack by NATO forces on our post will have serious repercussions as they without any reasons attacked on our post and killed soldiers asleep," said a senior Pakistani military officer, requesting anonymity.

NATO supplies blocked 

NATO supply trucks and fuel tankers bound for Afghanistan were stopped at Jamrud town in the Khyber tribal region near the city of Peshawar hours after the raid, officials said.

"We have halted the supplies and some 40 tankers and trucks have been returned from the check post in Jamrud," Mutahir Zeb, a senior government official, told Reuters.

Another official said the supplies had been stopped for security reasons.

"There is possibility of attacks on NATO supplies passing through the volatile Khyber tribal region, therefore we sent them back toward Peshawar to remain safe," he said.

The border crossing at Chaman in Baluchistan was also closed, Frontier Corps officials said.

Pakistan is a vital land route for 49 percent of NATO's supplies to its troops in Afghanistan, a NATO spokesman said.

A similar incident on Sept 30, 2009, which killed two Pakistani troops, led to the closure of one of NATO's supply routes through Pakistan for 10 days.

NATO apologized for that incident, which it said happened when NATO gunships mistook warning shots by the Pakistani forces for a militant attack.