The Pakistani Frontier Corps has been heavily infiltrated and influenced by Taliban militants, according to classified US documents appeared in a daily in London on Sunday.
There are "box loads" of reports of the troops joining militants in attacks on coalition forces, the classified US 'after-action' reports, compiled following clashes on Pakistan-Afghan border, said.
"The United States and NATO have substantial information on this problem. It's taking place at a variety of places along the border with the Frontier Corps giving direct and indirect assistance," the report in The Observer quoted an unnamed American official as saying.
"I'm not saying it is everyone. There are some parts that have been quite helpful. But if you have seen the after-action reports of their involvement in attacks along the Afghan border you would appreciate the problem."
The US documents reportedly describe the direct involvement of Frontier Corps troops in attacks on the Afghan National Army and coalition forces, and also about detailed attacks launched so close to Frontier Corps outposts that Pakistani co-operation with the Taliban is assumed.
The report quoted an unnamed source as saying: "The reality on the ground is that there are units so opposed to what the coalition is doing and so friendly to the other side that when the opportunity comes up they will fire on Afghan and coalition troops. And this is not random. It can be exceptionally well co-ordinated."
It quoted another unnamed source who had seen the documents as describing an attack last year where two Frontier Corps outposts appeared to have been directly involved in firing on Afghan forces before a militant attack.
The report said that Frontier Corps personnel had in the past been implicated in the past in murdering US and Afghan officers.
As the most high-profile case, it cited the example of Frontier Corps member killing Major Larry J Bauguess during a border mediation meeting.
James Appathurai, a NATO spokesman, said: "The real concern is that extremists in Pakistan are getting safe havens to rest, recuperate and retool in Pakistan and come across the border. The concerns have been conveyed to the Pakistan authorities."