The Pentagon insisted Wednesday that a NATO air strike that killed 24 Pakistani soldiers near the Afghan border was in no way a deliberate US attack on Pakistan.

The incident over the weekend infuriated Islamabad, with Pakistani officials saying the air strike was unprovoked and possibly a premeditated act of aggression.

But Pentagon officials said a military investigation would uncover exactly what took place on Saturday along the Afghan-Pakistani border and maintained that the United States had not set out to kill Pakistani troops.

"Whatever happened over the weekend on the border with Afghanistan and Pakistan was a tragedy. We've expressed regret for it as well as condolences for the loss of life.

"In no way, shape or form can this be construed as an intentional attack on Pakistan by the United States," US Defense Department spokesman George Little told reporters.

"That is simply incorrect."

The precise sequence of events surrounding the incident remain in dispute. While Islamabad says there was no provocation to justify firing on two Pakistani outposts, Afghan and Western officials have reportedly accused Pakistani forces of firing first.

US military officers have previously accused the Pakistani military of turning a blind eye to cross-border rocket attacks by Haqqani insurgents who have target American troops from positions near Pakistan's Frontier Corps posts.

On Tuesday, Pakistani Major General Ishfaq Nadeem briefed local journalists on the air strikes, saying the air assault was spread over a period of two hours despite Pakistani protests to the Americans.

In the aftermath of Saturday's strike, Pakistan has refused to attend a conference in Bonn designed to lay the ground for an eventual peace settlement in Afghanistan.

Pentagon spokesman Captain John Kirby acknowledged that relations between Pakistan and the United States faced unprecedented difficulties but that Washington was determined to shore up the uneasy partnership.

"Everybody recognizes how tough the situation is right now. The relationship is going through a very difficult period," Kirby told reporters.

"But I also can tell you that everybody on the US side, particularly the military and this department... is committed to having a good, strong, strategic relationship with Pakistan that's productive for both sides.

"That's hard, that's very hard to do."