Afghan officials say NATO and Afghan forces patrolling near the Pakistan border came under fire before they called in the NATO airstrikes that killed 24 Pakistani soldiers on Saturday.
Sunday's account by unnamed 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.
Rasmussen told Pakistani Prime Minister Yousuf Raza Gilani that the deaths of the Pakistani troops were "unacceptable and deplorable."
Earlier Sunday, Pakistan's Foreign Minister Hina Rabbani Khar told U.S. Secretary of State Hillary Clinton the air raid was not acceptable and demonstrated a complete disregard for human life. Clinton responded by saying she was deeply saddened, and she promised to work with Pakistan on the issue.
Pakistan has reacted strongly 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.
In the port city of Karachi, hundreds of angry protesters gathered outside the American consulate Sunday, shouting "Down with America" and other anti-American slogans.
Meanwhile, the nation's army chief, General Ashfaq Pervez Kayani, was among those attending funerals of the victims. Their coffins, draped in green and white Pakistani flags, are being airlifted to their respective hometowns.
Prime Minister 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 also is reexamining its decision to attend a major Afghanistan peace conference in Bonn next month.