The United States should apologize to Pakistan if an official investigation shows weekend NATO airstrikes that killed 24 Pakistani troops were "a mistake," a key US senator said Tuesday.

Senate Intelligence Committee chair Dianne Feinstein's comments came as her panel was about to hear behind closed doors from Lieutenant General Ronald Burgess, director of the US Defense Intelligence Agency, on the attacks.

"I'm one who believes that, if we made a mistake, we should admit it, we should apologize," Feinstein, a Democrat from California, told AFP.

But "if we didn't make a mistake, and the other side was the first one to fire on us, we're going to respond," she said.

Pakistan on Tuesday decided to boycott a key international conference on Afghanistan next month, widening its protest over the cross-border NATO strikes and worsening a deep crisis in US ties.

The Pakistani cabinet took the decision a week before the December 5 talks in the German city of Bonn, leaving open the possibility it could yet reverse course should Islamabad win concessions in the interim.

Pakistan has already closed the Afghan border to NATO convoys, a lifeline for 140,000 foreign troops in Afghanistan, ordered American personnel to vacate an air base reportedly used by CIA drones and ordered a review of the alliance.

Feinstein said "there's a very mixed view of what happened, so I hope General Burgess will be able to give us a fuller picture of exactly what happened."

"The first thing is: who fired first, what the nature of the attack was, who called in the NATO-US operation and why, were the Pakistanis really asleep?" said Feinstein.

Pakistan strike probe report due next month: US
Kabul (AFP) Nov 28, 2011 - A US-led investigation into a NATO air strike that killed 24 Pakistani troops near the Afghan border will report its initial findings by December 23, officials said Tuesday.

The chief of US Central Command, which oversees US forces in Afghanistan and the Middle East, appointed Brigadier General Stephen Clark, a one-star air force general, to lead the investigation, the US military announced.

The probe is expected to provide an initial report by December 23, it added.

Pakistan has reacted to Saturday's air strike with fury, cutting off crucial supply routes to NATO forces in Afghanistan, and ordering US personnel to vacate an air base reportedly used by CIA drones and a review of US relations.

Clark will lead the investigation with input from NATO and its International Security Assistance Force, which has 130,000 troops in Afghanistan in addition to an extra 10,000 American forces operating under separate command.

The Afghan and Pakistani governments are also being invited to take part, despite Pakistan's furious response to the attack.

"It is USCENTCOM's intent to include these government representatives to the maximum extent possible to determine what happened and preclude it from happening again," the US military said.

"The investigation team will focus their efforts on the facts of the incident and any matters that facilitate a better understanding of the circumstances surrounding the deaths and injuries of the Pakistani forces."

ISAF sent an initial assessment team to the border over the weekend.

A Western military official in Kabul, speaking on condition of anonymity, said the probe team had yet to arrive in Afghanistan but insisted its findings would be reported "way quicker" than initially expected.

The source said it was not unusual for US Central Command to carry out this kind of investigation rather than ISAF, which usually undertakes probes into incidents such as civilian casualties.

ISAF refused to comment when asked whether US Special Forces had been operating in the area when the air strikes were called in.

Islamabad insists that the air strikes were unprovoked, but Afghan and Western officials have reportedly accused Pakistani forces of firing first.

Before Saturday's attack, US military officers had been working to shore up cooperation with Pakistani forces along the Afghan border.

Communication between units on the border virtually broke down in the aftermath of a US raid in May that killed Al-Qaeda chief Osama bin Laden at his Pakistani compound, sending US-Pakistani relations into free fall.