In a landmark speech on counter-terrorism "ethics", Obama aide John Brennan insisted the program, the source of sharp tensions between Washington and Islamabad, 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.

US officials have generally been loath to publicly discuss the secretive CIA program, which has seen scores of what the US government says are Al-Qaeda suspects killed 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. AFP