Pakistan gives America two-week ultimatum to abandon 'secret' airbase and closes border: Hundreds of Afghan-bound supply trucks line up as tensions mount

As Pakistan army chief General Ashfaq Pervez Kayani attended the funerals of the victims, including a major, the U.S. sought to minimize the fallout from the crisis, which plunged Washington's already troubled relationship with Islamabad to an all-time low.

Pakistan also ordered the U.S. to vacate an airbase that is used by American drones to target al-Qaida and Taliban militants in the country's tribal region along the Afghan border.

There are forces working against a total rupture in the relationship.

Pakistan still relies on billions of dollars in American military and civilian aid, and the U.S. needs Islamabad's help to push Afghan insurgents to engage in peace talks.

But tensions could rise further if militants unleash attacks against the stranded trucks ferrying NATO supplies to Afghanistan.

The attack is the worst single incident of its kind since Pakistan uneasily allied itself with Washington in the days immediately following the September 11 attacks on U.S. targets.   

A spokesman for NATO's International Security Assistance Force (ISAF) said he believed aircraft that had been called in to provide air support for ground troops was responsible for the Pakistani casualties, guardian.co.uk reports.

NATO has called the attack a 'tragic unintended incident.'