But senior State Department troubleshooter Nicholas Burns said Washington would retain the option of targeting Osama bin Laden's terror group in Pakistani-Afghan border areas in some circumstances.

"We want to respect the sovereignty of the Pakistani government ... we want to work with the Pakistanis," Burns, under secretary of state for political affairs, told a hearing of the Senate Foreign Relations Committee.

"Are there any scenarios under which the United States might take its own action when we are dealing with Osama bin Laden and al-Qaeda? We can foresee some such scenarios," he said.

"But it's always going to be our preference to work with (Pakistan) ... we are partners of them, we don't want to complicate their internal politics needlessly."

Burns was replying to a question from Virginia Senator Jim Webb, who expressed concerns about the political impact of any US action in the tribal areas on President Pervez Musharraf, a key ally in the US "war on terror."

"We would have the potential of causing a ripple effect throughout the country which could truly destabilize the central government," Webb said.

Burns appeared before the committee several days after Pakistan reacted angrily to threats of action against militant targets in tribal territories, where US intelligence says al-Qaeda and the Taliban are regrouping.

Pakistani foreign ministry spokeswoman Tasnim Aslam said any such action would be "unacceptable," counterproductive and fan public anger.

The White House's top counter-terrorism official Frances Townsend on Sunday caused a stir by refusing to rule out a military incursion into remote Pakistani regions close to the border with Afghanistan.

"There are no tools off the table, and we use all our instruments of national power to be effective," Townsend, assistant to the president for homeland security and counterterrorism, told CNN.

Despite praising Pakistan for its increased military action in the tribal areas in recent weeks, Burns did say however that more needed to be done in the regions of North and South Waziristan.

"We would like to see a more sustained and effective effort by the Pakistani government to defeat terrorist forces on its soil," said Burns.