The already tenuous relationship between Pakistan and the United States has been plunged into another crisis, following NATO strikes on two military outposts in northwest Pakistan, on Saturday. U.S. President Barack Obama has vowed a full investigation into the attack that has allegedly killed 28 Pakistani troops.

Hillary Clinton, U.S. Secretary of State, and Leon Panetta, U.S. Defense Secretary, in a joint statement, offered their deepest condolences for the loss of life in the cross-border incident in Pakistan, the Associated Press reported.

Their statement stressed on the importance of the U.S.-Pakistan partnership for the mutual interests of people of both the countries. The U.S. leaders pledge to remain in close contact with their Pakistani counterparts going forward as we work through this challenging time, the statement concluded.

Islamabad, on the other hand, called the attack a grave infringement of the country's sovereignty and denounced it as the worst of its kind since Pakistan and the U.S. reluctantly came together, following the Sept. 11, 2001 attacks on the World Trade Center in America.

A NATO spokesman said it was likely that allied airstrikes had caused Pakistani casualties and confirmed that an investigation had been ordered.

Pakistan Retaliates

Meanwhile, the attack has prompted Islamabad to retaliate by shutting down vital NATO supply routes into Afghanistan and demand Washington remove U.S. forces from an air base used for CIA drone strikes on militants.

A meeting of the cabinet's defense committee, convened by Pakistan's Prime Minister, Yousuf Raza Gilani, decided to close with immediate effect NATO/ISAF logistics supply lines, according to a statement issued by the Prime Minister's office.

The U.S. has also been given 15 days to vacate the Shamsi Air Base in Baluchistan, which is being used by the Americans to launch drone strikes. The location is at the heart of a long-standing dispute between the two countries.

According to Reuters, Pakistan's Foreign Office said it would take up the matter in the strongest terms with NATO and the United States, while the Chief of Army Staff, General Ashfaq Pervez Kayani, said steps would be taken to respond to this irresponsible act.

Military officials believe that 28 soldiers died and a further 11 were wounded in the attack.