By Nicci Smith and Miles Amoore

The Sunday Times

Pakistan gave the go-ahead to American airstrikes last weekend that inadvertently killed 24 of their own troops, according to new claims from US officials.

The account is the latest twist in the blame game surrounding the worst friendly fire incident in the history of the 10-year war in Afghanistan, an event that has plunged America’s already precarious relations with Pakistan into a new crisis.

US officials speaking to The Wall Street Journal said that an Afghan-led assault force that included American commandos came under fire from a camp in Pakistan’s Mohmand tribal region, a lawless border area that adjoins Kunar province in Afghanistan.

Afghan intelligence said the force was searching for a senior insurgent commander, but they stumbled onto a unit of Pakistani soldiers dressed in plain clothes, who shot at them first.

“The reports show that the Americans thought these guys were insurgents, so they opened fire on them,” a senior intelligence official told The Sunday Times.

The “militants” now appear to have been Pakistani border troops who had established a temporary base.

An initial American account based on interviews with the commandos claims the team requested aerial back-up to strike the camp, contacting a joint border-control centre to establish whether Pakistani forces were in the area. The centre is manned by US, Afghan and Pakistani officials to coordinate information to prevent clashes.

When called, the Pakistani officials at the centre allegedly said they had no military forces in the area, clearing the way for the airstrikes.

The US has acknowledged mistakes were made on both sides. To protect troops, officials working in the centre need to know whether NATO forces are planning operations, but no advance warning had been sent of the 26 November operation.

US officials have been reluctant in the past to share information for fear of it leaking out to insurgents.

Washington has expressed its regret over the “ tragic accident”, but pointedly stopping short of an apology.

But its condolences have been rebuffed by an unforgiving Pakistan, where the military and government have united to angrily condemn the incident as an “unprovoked act of blatant aggression.”

The Pakistani military categorically denied the latest American version of events, claiming Pakistan had been fed “wrong information” and was contacted only after the strike began.

“Without getting clearance from the Pakistan side, the post had already been engaged by US helicopters and fighter jets. Pakistan did not have any prior information about any operation in the area,” said an official.

As tension worsened, army chief, General Ashfaq Kayani on Friday changed the rules of engagement to allow Pakistani border troops to return fire if attacked, without prior permission.

“I do not want there to be any doubt in the minds of any commander at any level,” he said. “In case of any attack, you have complete liberty to respond forcefully, using all available resources.”

Pakistan’s story, according to Major General Ishfaq Nadeem, the director general of military operations, began with a strange report of firing from the Pakistani side of the border.

Shortly after midnight, a NATO sergeant called a Pakistani major at one of the border control centres to complain that US Special Forces had come under “indirect fire” from a Pakistani border checkpoint at Gora Parai, Mohmand.

But seven minutes later, the sergeant called again to say that NATO forces had engaged a different checkpoint, known as Volcano, around ten miles south.

Pakistan has fiercely denied that there was any firing from either checkpoint and says the first round of firing from Apache helicopters against the Volcano checkpost occurred between 12.15 and 12.30am.

When a second Pakistani border post a few hundred yards north, codenamed Boulder, saw the attack, it retaliated with anti-aircraft guns. As a result, it also came under fire.

News of the attacks was relayed up the Pakistani army’s chain of command, who spoke with an International Security Assistance Force [ISAF] commander in Afghanistan and asked for the NATO helicopters to stop.

Soon after 1am the helicopters “pulled back”, allowing Pakistani reinforcements to rush to the two checkposts.

But according to eyewitnesses, 15 minutes later the NATO aircraft began, inexplicably, to fire again in “intermittent attacks” for a second time.

Nadeem said the Pakistani troops fired “26 rounds of artillery air bursts” as “warning fire”, but when this second exchange ended at 2.15 am the two checkposts had been obliterated.

The conflicting accounts have been exacerbated by a dispute over the exact location of the hostile, mountainous border, known as the Durand Line.

Afghan villagers and officials claim that, over the past six months, Pakistani border patrols have increasingly strayed into Afghanistan, and have fired more artillery shells, forcing civilians to flee.

Others have blamed the recent killings on the close relationship between elements of Pakistan’s security forces, including the Frontier Corps, and insurgents based near the border.

Washington has pledged a full investigation into the incident, due to be concluded on 23 December, but this may be hampered by Pakistan’s refusal to take part.

The timeline may also be too late to turn the rising tide of anti-American opinion among the Pakistani public or fix the diplomatic fallout.

Pakistan’s government has already pulled out of a security conference on Afghanistan in Bonn, Germany, next week, although it is still mulling whether to send an ambassador-level delegation.

Within hours of the strike Pakistan closed the NATO supply routes from the port of Karachi up to the Afghan border crossing, shutting off roughly 60 percent of vital supplies to allied troops.

Last week US officials also began to leave the Shamsi airbase, the unofficial launchpad for drones targeting militants in Pakistan’s tribal areas, after Islamabad issued a 15 day ultimatum to vacate the site.