They are robots in the sky and some say they are revolutionizing the way the United States wages wars. Drones are playing a growing role in the U.S. military.

It is estimated that there are 10,000 unmanned aerial vehicles in the U.S. military's arsenal, in addition to an undisclosed number operated by the CIA - including one that recently killed Abu Yahya al-Libi, al-Qaida's number two leader.

Pakistan objects to the use of drones over its territory. But U.S. Defense Secretary Leon Panetta has given no indication Washington will stop using them against terrorists.

"We made it clear to the Pakistanis that United States of America is going to defend ourselves against those that would attack us, and we have done just that, we have gone after their leadership and we have done it effectively," said Panetta.

Drones are relatively cheap to operate. Their strikes are precise. And they entail no risk to the pilots who operate them from U.S. bases thousands of kilometers away.

At a time of shrinking budgets and growing war fatigue among the American public, the Obama administration has made unmanned aerial vehicles a central component of its new defense strategy.

Michele Flournoy is a former top Pentagon official and an architect of that strategy.

"The whole realm of unmanned systems is going to revolutionize the force over time," said Flournoy. "We are still in the process of understanding what those systems bring in terms of new ways of operating, new ways of working as a military."