US Secretary of State John Kerry on Friday defended the legality of US drone strikes in the "war" against Al-Qaeda and the Taliban, days after such an unmanned aircraft killed a top Pakistani Taliban.

"Despite our first preference for detention and prosecution of terrorists -- that's our first preference -- sometimes, lethal action is necessary in order to protect US lives," the top US diplomat told reporters alongside his German counterpart Guido Westerwelle.

"Our actions are legal. We were attacked on 9/11. Within a week, the United States Congress overwhelmingly authorized the use of force," Kerry said. "Under domestic law and international law, the United States is at war with Al-Qaeda and the Taliban and their associated forces."

Kerry's comments took up arguments articulated by President Barack Obama, who on May 23 laid out new guidelines for drone strikes while mounting a firm defense of the covert drone war as legal.

The guidelines state that drone strikes can only be used to prevent imminent attacks and when the capture of a suspect is not feasible, and if there is a "near certainty" that civilians will not be killed.

On Wednesday, a US drone strike killed the deputy chief of the Pakistani Taliban in the country's lawless tribal northwest, officials said, dealing a major blow to the militant network.

Waliur Rehman, the number two in the Tehreek-e-Taliban Pakistan (TTP) faction, died along with at least five others when an unmanned US drone fired two missiles on a house in North Waziristan district.

The White House has not confirmed the killing, but on Friday Pakistan's incoming prime minister, Nawaz Sharif, condemned the strike.