A top US official has defended as legal and ethical its use of drone strikes to target terrorism suspects.

The officials comments on Monday by White House counter-terrorism adviser John Brennan was the most direct acknowledgement yet of the clandestine programme.

"Yes, in full accordance with the law - and in order to prevent terrorist attacks on the United States and to save American lives - the United States government conducts targeted strikes against specific al-Qaeda terrorists, sometimes using remotely piloted aircraft, often referred to publicly as drones," John Brennan said in a speech at the Woodrow Wilson Centre in Washington.

Brennan said President Barack Obama had asked the administration to be more open about the drone programme.

He defended the attacks as legal and noted broad discussion about the use of the strikes.

"We discuss. We debate. We disagree," he said. "We consider the advantages and disadvantages of taking action. We also carefully consider the costs of inaction and whether a decision not to carry out a strike could allow a terrorist attack to proceed and potentially kill scores of innocents."

'Not legal analysis'

The American Civil Liberties Union, a longstanding critic of the tactic, welcomed the acknowledgement of the programme but said Brennan did not provide sufficient legal justification.

"Mr Brennan supplies legal conclusions, not legal analysis," Deputy Legal Director Jameel Jaffer said in a statement.

"We continue to believe that the administration should release the Justice Department memos underlying the programme - particularly the memo that authorises the extra judicial killing of American terrorism suspects."

A suspected US drone strike most recently left four dead in Pakistan, and the issue has proved a major hurdle in relations with Islamabad.

Following an attack that left two dozen Pakistani soldiers dead last year, Islamabad has demanded a halt to the drone programme before reopening NATO supply routes through its territory.

Earlier this year, Obama was asked about the drone programme during a virtual town hall meeting and stressed that the US was "judicious" in its use of the technology.

Attorney General Eric Holder has offered a legal defence of the killing, last year in Yemen, of al-Qaeda cleric Anwar al-Awlaki, a US citizen.