US Defense Secretary Robert Gates declared Afghanistan the top US military priority but said US objectives there should be "limited."

"My own personal view is that our primary goal is to prevent Afghanistan from being used as a base for terrorists and extremists to attack the United States and our allies," he said.

"And whatever else we need to do flows from that objective," he told lawmakers in his first appearance before the Senate Armed Services Committee as President Barack Obama''s defense secretary.

His comments marked a significant narrowing of US ambitions even as the United States prepares to nearly double the size of its forces in Afghanistan in response to an unraveling security situation.

"There is little doubt that our greatest military challenge right now is Afghanistan," Gates said in his opening statement. "President Obama has made it clear that the Afghanistan theater should be our top military priority."

Pakistan on Wednesday hit back against Gates, saying US missile strikes inside its borders were "counter-productive" to anti-terrorism efforts.

"Our policy remains unchanged and we believe drone strikes are counter-productive," foreign ministry spokesman Mohammad Sadiq told AFP.

He was speaking in response to a statement from Gates that the United States would "go after Al-Qaeda wherever Al-Qaeda is" and affirming that the new US administration''s position had been transmitted to the Pakistani government.

Gates told lawmakers that the bulk of a 30,000-troop buildup requested by the US commander in Afghanistan, General David McKiernan, could be in place by mid-summer but bases must be expanded to receive the full complement of additional forces.

"But I would be very skeptical of additional forces levels, American force levels, beyond what General McKiernan has already asked for," he said.