As Pakistan simmered over the killings, Wall Street Journal quoted US officials in Kabul as saying that insurgents may have been firing into Afghanistan near the Pakistani border which prompted coalition forces to strike back.

The account challenges Islamabad's claims that the attacks, which have plunged US-Pakistan ties to new lows, were unprovoked and risked stoking fresh tensions as the incident has left US-Pakistan relations in tatters.

Afghan officials working in the border area where the attack took place said that the joint forces was targetting Taliban militants in the area when it received fire from the Pakistani military outpost.

That prompted the coalition forces to call for an air attack on the Pakistani forces, the Afghan officials said and also claimed that Pakistani officials were informed of the operation before it took place.

"There was firing coming from the position against Afghan army soldiers who requested support and this is what happened," WSJ quoted an Afghan official as saying as the top US commander Gen. John Allen held a meeting in Kabul to discuss the incident.

The Afghan officials said the government believes that the fire came from the Pakistani base and not from insurgents operating nearby. A view bolstered by a western official who also said that coalition forces were fired on from a Pakistani army base.

But, the Pakistani army has questioned why NATO undertook a sustained two-hour attack on the well-known border positions in which he said helicopters and fighter jets were used.

"No first fire came from Pakistani troops," the paper quoted a senior Pakistani military official as saying. "But they did respond in self defence after NATO helicopter gunships and jet fighters carried out unprovoked firing."

Military officials in Kabul said insurgents in Pakistan have also used empty Pakistani border posts to stage attacks, which may have been the working assumption of the coalition forces who called in the air strike.

US officials said the units believed they were responding to incoming fire from Pakistan's side of the border.

"They believed they were coming under attack from that side of the border," a senior US official said, although investigators have yet to pinpoint the precise source of firing.

Pakistani military disputes this version of events saying that the two border posts attacked without warning at 2 AM while most of the around 50 soldiers were sleeping.

The Pakistani officials claim that NATO helicopters and jets even attacked Pakistani military forces sent in as back-up during the two-hour assault.

They said they have increased the number of soldiers at the border posts in the Mohmand area this year to wipe out Taliban in the area.

At yesterday's security meeting in Kabul, the Afghan leaders have made public the second phase of plans for coalition forces to cede security control to Afghan forces.

The new plan includes six of the country's 34 provinces including Kabul, seven major cities and dozens of districts including Jalalabad and Helmund.

If the transition proves success, it will put Afghan forces in the lead in protecting more than half of the country.