US President Barack Obama has ordered more openness about the secretive drone war pounding Al-Qaeda in Pakistan, a top official said, mounting the firmest defense yet of the program.

In a landmark speech on counter-terrorism "ethics," Obama aideJohn Brennan insisted the program was legal, ethical, proportional and saved US lives.

The top counter-terrorism official gave the most detailed justification and description of the drone program yet, on the eve of the anniversary of the killing of Al-Qaeda leader Osama bin Laden in a raid in Pakistan by elite special forces.

US officials have generally been loath to publicly discuss the secretive CIA program, the source of sharp tensions between Washington and Islamabad. Drones have killed scores of what the US government says are Al-Qaeda suspects in tribal areas of Pakistan and some other nations.

"Broadly speaking, the debate over strikes targeted at individual members of Al-Qaeda has centered on their legality, their ethics, the wisdom of using them, and the standards by which they are approved," Brennan said.

"The United States government conducts targeted strikes against specific Al-Qaeda terrorists, sometimes using remotely piloted aircraft, often referred to publicly as drones," Brennan said in a speech to the Woodrow Wilson International Center for Scholars in Washington.

"I'm here today because President Obama has instructed us to be more open with the American people about these efforts."

Brennan said the strikes had been certified as complying with US law by US Attorney General Eric Holder, among other top officials.

"The constitution empowers the president to protect the nation from any imminent threat of attack," Brennan said, adding that Obama's action was also authorized by measures passed by Congress after the September 11 attacks in 2001.

He argued that the strikes were ethical, proportional and conformed to US efforts to spare innocent civilians from being caught up in the crossfire.

"It is hard to imagine a tool that can better minimize the risk to civilians than remotely piloted aircraft," Brennan said, in a speech briefly interrupted by a Code Pink anti-war protestor.

"There is absolutely nothing casual about the extraordinary care we take in making the decision to pursue an Al-Qaeda terrorist, and the lengths to which we go to ensure precision and avoid the loss of innocent life."

During a press conference with senior Philippine officials at the State Department, US Defense Secretary Leon Panetta said he would let Brennan's speech speak for itself.

But Panetta added: "We have made clear that we're going to do everything we can to defend this country using every means possible and the means we use are those that we feel are the most effective to go after Al-Qaeda."