Pakistan has denied reports its troops opened fire first on NATO and Afghan forces, triggering lethal cross-border airstrikes that killed 24 Pakistani soldiers.

Pakistan's army spokesman Major General Athar Abbas said Monday claims that Pakistani troops initiated the firefight are not true.

Afghan officials said Sunday NATO and Afghan forces patrolling near the Pakistan border came under fire before they called in the NATO airstrikes that killed the Pakistani soldiers Saturday.

Sunday's account by unnamed Afghan officials contradicts Islamabad's claims that the attack on two Pakistani army bases was unprovoked.

NATO and U.S. officials responded quickly to try to minimize the diplomatic repercussions of the attack. NATO chief Anders Fogh Rasmussen on Sunday promised a full investigation into the “tragic, unintended” deaths.

Earlier Sunday, Pakistan's Foreign Minister Hina Rabbani Khar told U.S. Secretary of State Hillary Clinton the air raid demonstrated a complete disregard for human life. Clinton said she was deeply saddened by the events and promised to work with Pakistan on the issue.

Pakistan responded to the airstrikes by shutting down all NATO supply lines through its territory to Afghanistan and ordering the U.S. to vacate an air base in southwestern Baluchistan province within 15 days.

Prime Minister Yousuf Raza Gilani and top leaders said in a statement Saturday the Pakistani government “will revisit and undertake a complete review of all programs, activities and cooperative arrangements with US/NATO/ISAF.” The statement called for strong and urgent action against those responsible for the deadly incident.

Pakistan says it is reexamining its decision to attend a major Afghanistan peace conference in Bonn next month.