The United States will have to keep up an open-ended drone war against Al-Qaeda militants in Pakistan and elsewhere to prevent another terror attack on America, Defense Secretary Leon Panetta said.

The assassination of Al-Qaeda figures in Pakistan, Yemen and Somalia with unmanned, robotic aircraft has provoked widespread criticism from human rights groups and some US allies, but Panetta said the US campaign has been effective.

Asked if the CIA "targeted killings" should be curtailed in coming years, Panetta told AFP in an interview Friday that there was still a need to continue the drone strikes more than a decade since the attacks of September 11, 2001.

"I think it depends on the nature of the threat that we're confronting. We are in a war. We're in a war on terrorism and we've been in that war since 9/11.

"The whole purpose of our operations were aimed at those who attacked this country and killed 3,000 innocent people in New York as well as 200 people here at the Pentagon," said Panetta, who is days away from retiring as Pentagon chief.

Before taking over as defense secretary, Panetta oversaw a dramatic increase in drone raids in Pakistan as head of the CIA from 2009-2011.

"I think we had a responsibility to use whatever technology we could to be able to go after those who not only conducted that attack but were planning to continue to attack this country," he said.

"It's been an important part of our operations against Al-Qaeda, not just in Pakistan, but also in Yemen, in Somalia and I think it ought to continue to be a tool we ought to use where necessary," he said.

"And also where we can use it in conjunction with other countries that are pursuing the same goal," said Panetta, citing Yemen's strong support for drone strikes.

The CIA drone bombing raids, by Predator and Reaper aircraft armed with Hellfire missiles, have caused an unknown number of civilian casualties and prompted accusations that Washington is carrying out extrajudicial killings in the shadows with no genuine oversight by courts or lawmakers.

Panetta, who as CIA director presided over the successful raid that killed Al-Qaeda founder Osama bin Laden, said the campaign still needed to be regularly reviewed but did not say he favored turning over the spy agency's drone war to the military.

Some critics have called for giving the US military authority over the drone air strikes, which would require openly reporting every operation.

"Having said that, we always need to continue to look at it, to make sure we develop the right standards, that we're abiding by the laws of this country, that we're doing it in a way that hopefully can be a little more transparent with the American people."

But he said "to protect this country" it was not enough to have operations carried out openly by the military.

Secret action led by the CIA was also needed "when you got those kind of operations where, because of the nature of the country you're in or the nature of the situation you're dealing with, it's got to be covert."

According to the London-based Bureau of Investigative Journalism, between 2,627 and 3,457 people have been reportedly killed by US drones in Pakistan since 2004, including between 475 and nearly 900 civilians.

The covert strikes are publicly criticised by the Pakistani government as a violation of sovereignty but American officials believe they are a vital weapon in the war against Islamist militants.

Few of the victims are publicly identified.