Saturday, 24 Pakistani soldiers at two posts along the frontier were killed.
Afghan officials said their troops had come under fire from the direction of those posts. The officials, who spoke on condition of anonymity because of the sensitivity of the issue, said it's unclear who attacked the Afghan troops.
Pakistan's army claims the attack was unprovoked and insists there was no militant activity in the area.
Meantime, Islamabad has closed its border to the trucks that deliver supplies to coalition troops and is demanding the U.S. vacate a base thought to be used for launching drones.
NATO has said it is likely that its aircraft carried out the attack that caused Pakistani casualties and is conducting an investigation to determine the details. The Pakistan-Afghanistan border is disputed and not marked in many areas, adding to the difficulty.
The incident has driven to new lows the United States' already tattered alliance with Pakistan, a relationship that is vital to winding down the 10-year-old Afghan war.
Secretary-General Anders Fogh Rasmussen, NATO's top official, offered his "deepest condolences" and said the coalition was committed to working with Pakistan to "avoid such tragedies in the future."
"We have a joint interest in the fight against cross-border terrorism and in ensuring that Afghanistan does not once again become a safe haven for terrorists," Rasmussen said in Brussels.
The Associated Press contributed to this report.