A senior Pakistani Army official has said a Nato cross-border air attack that killed 24 soldiers was a deliberate, blatant act of aggression, hardening Pakistan's stance on an incident which could hurt efforts to stabilise Afghanistan.

In a briefing to editors carried in local newspapers on Wednesday, major-general Ishfaq Nadeem, director general of military operations, also said Nato forces were alerted they were attacking Pakistani posts, but helicopters kept firing.

"Detailed information of the posts was already with ISAF (International Security Assistance Force), including map references and it was impossible that they did not know these to be our posts," The News quoted Nadeem as saying in the briefing held at Army headquarters on Tuesday.

The Nato attack on early on Saturday shifted attention away from Pakistan's widely questioned performance against militants who cross its border to attack US-led Nato forces in Afghanistan, and has given the military a chance to reassert itself.

On Tuesday, Pakistan pulled out of an international conference on Afghanistan, an angry riposte after the attack by Nato plunged the region deeper into crisis.

Islamabad's decision to boycott next week's meeting in Bonn, Germany, will deprive the talks of a key player that could nudge Taliban militants into a peace process as Nato combat troops prepare to leave Afghanistan in 2014.

Secretary of state Hillary Clinton said on Wednesday she regretted Pakistan's decision to boycott next week's international conference on Afghanistan but hoped to secure Islamabad's cooperation in future.

"Nothing will be gained by turning our backs on mutually beneficial cooperation. Frankly it is regrettable that Pakistan has decided not to attend the conference in Bonn," Clinton told a news conference in South Korea.

The army, which has ruled the country for more than half of its history and sets security and foreign policy, faced strong criticism from both the Pakistani public and its ally, the United States, after the secret US raid that killed Osama bin Laden.

The Al Qaeda leader had apparently been living in a Pakistani garrison town for years before US special forces found and killed him in a unilateral operation.

Pakistanis criticized the military for failing to protect their sovereignty, and angry US officials wondered whether some members of military intelligence had sheltered him.

The army seems to have regained its confidence, and anti-Nato protests suggest it has won the support of the public in a country where anti-American sentiment runs high even on rare occasions when relations with Washington are healthy.

Exactly what happened at the Pakistani posts along an unruly and poorly defined border is still unclear. Nato has promised to investigate.

A Western official and an Afghan security official who requested anonymity said Nato troops were responding to fire from across the border. Pakistan said earlier the attack was unprovoked.

Both the Western and Pakistani explanations are possibly correct: that a retaliatory attack by Nato troops took a tragic, mistaken turn in harsh terrain where differentiating friend from foe can be difficult.

Nadeem was adamant that all communications channels had informed Nato that it was attacking Pakistani positions.

"They continued regardless with impunity," The News quoted him as saying.