Pakistani officials gave the go-ahead to a NATO air strike that killed 24 Pakistani troops, unaware that their own forces were in the area, the Wall Street Journal reported on Friday quoting U.S. officials.

Last weekend's cross-border attack has caused public outrage in Pakistan, where the government has pulled out of next week's international conference on Afghanistan and threatened to end support for the U.S.-led war there if its sovereignty is violated again.

The U.S. officials, giving their first detailed explanation of the worst friendly-fire incident of the 10-year-old war, said an Afghan-led assault force that included U.S. commandos was hunting Taliban militants when it came under fire from an encampment along the border with Pakistan, the Journal said in an online report.

The commandos thought they were being fired on by militants but who turned out to be Pakistani military personnel who had established a temporary campsite, they were quoted as saying.

According to the initial U.S. account from the field, the commandos requested air strikes against the encampment, prompting the team to contact a joint border-control centre to determine whether Pakistani forces were in the area, a U.S. official said.

The border-control centre is manned by U.S., Afghan and Pakistani representatives. But the U.S. and Afghan forces conducting the November 26 commando operation had not notified the centre in advance that they planned to strike Taliban insurgents near that part of the border, the official said.

When called, the Pakistani representatives at the centre said there were no Pakistani military forces in the area identified by the commandos, clearing the way for the air strikes, the U.S. officials said.

Washington has called it a tragic accident and offered its condolences, promising a full investigation. It has not apologised.

There were lots of mistakes made, the newspaper quoted an official as saying.

The Journal said U.S. officials have in the past expressed reservations about notifying the Pakistanis about operations, concerned the missions' details could leak out.

It added that the officials cautioned that the preliminary account is based mainly on interviews with members of the commando team and could change as more information is gathered.

A formal report on the incident is due to be completed by U.S. military investigators by December 23.

(Writing by Jonathan Thatcher)