The United States defence secretary has said that even though Washington was working with Pakistan to to combat al Qaeda in its border areas, American forces should be ready to act "unilaterally" to take out terror targets.

"Al Qaeda [Images] training and reconstituting itself...it's one area where we need to see if we can get the Pakistanis to take it on, then to work with the Pakistanis to improve their capabilities or do things together," Robert Gates told the House Armed Services Committee.

"And then we need to be able to act unilaterally, if we have to, to make sure they don't come back at us again," he said.

Gates said he believed military operations to ensure that al Qaeda has no havens in Afghanistan were successful, but was "concerned" about "what al Qaeda is doing on the Pakistani side of the border".

The defence secretary, however, said the US would not be looking to add significant numbers of US forces to run operations in Pakistan.

"...in terms of the al Qaeda haven, it's not in Afghanistan where we would put additional troops if we wish to, but rather in Pakistan. And so, the issue is: How do we work with the Pakistanis to make them more effective and what can we do together or, perhaps, independently? But that's clearly not going to be major force movements across the border," Gates said.

"But there's no question that we have to worry about al Qaeda operating on the Pakistani side of the border," he said.