Kamal Hyder, Al Jazeera's correspondent in Islamabad, said that the US and Pakistan had had a very strong relationship under the previous administration dominated by supporters of Pervez Musharraf, the president.
Nielson-Green refused to comment on whether it was Pakistani soldiers who were firing on US-led forces but said video footage of the incident was in the process of being declassified.
A statement from the US military said the air and artillery assault was aimed at Taliban fighters nearby and that they had informed Pakistani authorities about the operation.
Geoff Morrell, a
Affront to sovereignty
In Pakistan, politicians and military officials issued a furious denunciation of the US response as people wounded in the attack were being treated in a
The soldiers were killed at a border post in the volatile Mohmand province, a tribal region in Pakistan opposite Afghanistan's Kunar province.
"The people involved in this incident … were firing on coalition forces and they were well inside of
"This is an absolutely baseless allegation or explanation. We have co-ordination, we have intelligence sharing. If there was some doubt about any post they should have informed us before taking up any strike," he said.
"We will wait for their reply. And whatever is their reply we would like them to consider it seriously."
This was "unprovoked and cowardly … we blame the collation forces … it has hit hard at the basis of our co-operation", he said.
Yousaf Raza Gilani,
"We will take a stand for sovereignty, integrity and self-respect and we will not allow our soil [to be attacked]," he told parliament.
A US military statement said pro-Taliban fighters were sighted in a wooded area near the border post and US-led forces struck with artillery fire.
Maulvi Omar, a Pakistani Taliban spokesman, said that his fighters had attacked US and Afghan forces as they were setting up a position on the Pakistan side of the border.
The incident came after Kabul and Western forces in Afghanistan raised doubts about Pakistan's efforts to negotiate pacts with tribal fighters to end violence on its side of the border.