Pakistan has denied reports that it opened fire first provoking the Nato air strike which killed 24 troops at a checkpoint on the Afghan border.
It follows claims by Afghan officials that Nato forces were retaliating for gunfire from the Pakistani side of the volatile border region on Saturday.
On Sunday Pakistan's army chief led mourners as those killed in the strike were buried at military headquarters.
Nato has apologised, calling it a "tragic unintended incident".
But the attack has heightened already tense relations between Pakistan, the US and Nato. It took place at two remote border posts in Pakistan's tribal district of Mohmand in the early hours of Saturday morning and recriminations have been pouring in response to the attack .
Unnamed Afghan officials quoted in The Wall Street Journal said that Saturday's attack was called in to shield Nato and Afghan forces who were under fire while targeting Taliban fighters. One official quoted in the paper says that Kabul believes the fire came from an army base.
"This is not true. They are making up excuses. What are their losses, casualties?" Pakistani army spokesman Maj-Gen Athar Abbas said in a text message in response to the allegations.
Maj-Gen Abbas has also said that the raid went on for more than an hour and continued even after local commanders contacted Nato telling them to stop the strike, according to the Associated Press news agency.
Pakistani officials have consistently maintained that there had been no militant activity in the area, and most of the Pakistani soldiers were asleep. They also said Nato had the grid references of the posts and therefore should not have fired.
'Grave infringement'
Pakistan's Prime Minister Yousuf Raza Gilani called it a "grave infringement of Pakistan's sovereignty" and officials responded by cutting key supply Pakistani lines to Nato in Afghanistan.
The BBC's M Ilyas Khan in Islamabad says that public anger has intensified amid growing demands from Pakistan's opposition parties to sever all ties with the US.
On Monday lawyers staged protests across the country and Reuters news agency reports that Pakistan's fuel suppliers have said that they will not resume supplies to Nato forces in Afghanistan any time soon.
Nato chief Anders Fogh Rasmussen said he had written to Mr Gilani to "make it clear that the deaths of Pakistani personnel are as unacceptable and deplorable as the deaths of Afghan and international personnel".
Nato has said it is investigating what happened.
In a joint statement, US Secretary of Defense Leon Panetta and Secretary of State Hillary Clinton, offered their condolences for the loss of life, backed the investigation into the incident and stressed the "importance of the US-Pakistani partnership, which serves the mutual interests of our people".
The night-time attack took place at the Salala checkpoint, about 1.5 miles (2.5 km) from the Afghan border, at around 02:00 on Saturday morning local time (21:00 GMT Friday).
The Pakistani army said helicopters and fighter aircraft hit two border posts, killing 24 people and leaving 13 injured. Local officials said the two posts were about 300m apart on a mountain top.
Military sources earlier told the BBC's Quentin Sommerville in Paktika province in Afghanistan that a US-Afghan special forces mission had been in the area, where they believed a Taliban training camp was operating.
They said the mission came under fire from a position within Pakistan, and they received permission from the headquarters of Nato's Isaf mission to fire back.
The incident looks set to deal a fresh blow to US-Pakistan relations, which had only just begun to recover following a unilateral US raid that killed Osama Bin Laden in Pakistan in May.
Pakistani troops are involved in fighting the Taliban in the crucial border region area. Hundreds of militants have been resisting attempts by the security forces to clear them from southern and south-eastern parts of the district.
BBC News - Pakistan denies firing provoked Nato border attack
Anyone else buy this?
