White House in first detailed comments on drone strikes
President Obama's counter-terrorism adviser has given the most detailed explanation so far of America's use of drones to kill members of al-Qaeda.
In a speech to a Washington think tank, John Brennan said the strikes were helping to win the war on the militant network.
President Barack Obama wanted to be more open about the practice, Mr Brennan added.
The comments come in the week marking a year since Osama Bin Laden's death.
BBC Washington correspondent Paul Adams says this is not the first time the Obama administration has confirmed the use of drone strikes.
'Disaster after disaster'
In January, the president did it himself, during a webchat. But our correspondent says Mr Brennan has gone further than anyone so far in laying out the rationale for a policy that remains controversial.
Mr Brennan said unmanned drone strikes were legal, ethical, necessary and proportional, overseen with what he called extraordinary care and thoughtfulness, especially when the target was an American citizen.
In his speech at the Woodrow Wilson International Center, he said al-Qaeda was losing badly.
For the first time since America's war on the organisation began, Mr Brennan said it was possible to envision a world in which the core of al-Qaeda was no longer relevant.
He added that drone strikes usually took place with the co-operation of the host government, in "full accordance with the law".
Such strikes are thought to have killed hundreds of militants in Afghanistan, Pakistan and Yemen.
But Mr Brennan also conceded that there had been civilian deaths as a result of some strikes.
Pakistan has previously demanded an end to US drone strikes on Pakistani soil.
Hina Shamsi, director of the American Civil Liberties Union, told the BBC's World Today programme that Mr Brennan's speech showed the US administration believed its authority went "far beyond what has been recognised under international law".
Ms Shamsi said: "We believe there are few things as dangerous as the proposition that the government should be able to kill people anywhere in the word, including citizens, on the basis of legal standards and evidence that are never submitted to the court either before or after the fact."
She added: "Anyone willing to trust President Obama with the power to secretly declare people as enemies of the state and order their extrajudicial killing should ask whether they would be willing to trust the next president with that dangerous power, and the next president after that."
'Disaster after disaster'
Mr Brennan also said that documents found at the compound where Osama Bin Laden was killed in Pakistan last year would go online later this week.
They were gathered by US Navy Seals during the raid on Bin Laden's hideout in Abbottabad on 2 May 2011.
The papers are said to include communication between Bin Laden and his associates, and his hand-written diary.
They are said to reveal that Bin Laden had considered changing al-Qaeda's name because so many of the group's senior operatives had been killed.
"In short, al-Qaeda is losing badly. And Bin Laden knew it. In documents we seized, he confessed to 'disaster after disaster'," Mr Brennan said.
"With its most skilled and experienced commanders being lost so quickly, al-Qaeda has had trouble replacing them."
Mr Brennan said the documents would be put online by the US Military Academy's Combating Terrorism Center.