President Barack Obama has confirmed for the first time that US drones have targeted Taliban and Al-Qaeda militants on Pakistani soil, a programme that has escalated under his administration.

The government in Islamabad, whose relations with Washington sank to an all-time low last year, appeared to shrug off the confirmation but made a rare public acknowledgement that the programme had "tactical advantages".

Asked about drones in a chat with web users on Google+ and YouTube, Obama said "a lot of these strikes have been in the FATA" -- Pakistan's semi-autonomous Federally Administered Tribal Areas on the Afghan border.

"For the most part, they've been very precise precision strikes against Al-Qaeda and their affiliates, and we're very careful in terms of how it's been applied," Obama said on Monday.

"This is a targeted focused effort at people who are on a list of active terrorists, who are trying to go in and harm Americans, hit American facilities, American bases, and so on."

He said that many strikes were carried out "on Al-Qaeda operatives in places where the capacities of that military in that country may not be able to get them", such as Pakistan's lawless tribal zone.

"For us to be able to get them in another way would involve probably a lot more intrusive military action than the ones we're already engaging in."

US officials say Pakistan's tribal belt provides sanctuary to Taliban fighting in Afghanistan, Al-Qaeda groups plotting attacks on the West, Pakistani Taliban who routinely bomb Pakistan and other foreign fighters.

According to an AFP tally, 45 US missile strikes were reported in Pakistan's tribal belt in 2009, 101 in 2010 and 64 in 2011.

The New America Foundation think tank in Washington says drone strikes in Pakistan have killed between 1,715 and 2,680 people in the past eight years.

Human rights campaigners have expressed deep concern over increased use of drone strikes. The State Department also confirmed it used surveillance drones to protect US diplomats in so-called "critical threat environments" overseas.

The United States had until now refused to discuss the strikes publicly, but the program has dramatically increased as the Obama administration looks to withdraw all foreign combat troops from Afghanistan by the end of 2014.

US diplomatic cables leaked by WikiLeaks in late 2010 showed that Pakistan's civilian and military leaders privately supported US drone attacks, despite public condemnation in a country where the US alliance is hugely unpopular.

"Notwithstanding tactical advantages of drone strikes, we are of the firm view that these are unlawful, counterproductive and hence unacceptable," foreign ministry spokesman Abdul Basit told AFP on Tuesday.

Relations between the United States and Pakistan deteriorated sharply in 2011, over the covert American raid that killed Al-Qaeda chief Osama bin Laden in May and US air strikes that killed 24 Pakistani soldiers in November.

Islamabad is now reviewing its entire alliance with the United States and has kept its Afghan border closed to NATO supply convoys since November 26.

It ordered US personnel to leave Shamsi air base in western Pakistan, widely believed to have been a hub for the CIA drone program, and is thought likely to only reopen the Afghan border by exacting taxes on convoys.

But analyst Imtiaz Gul, who has written extensively about Pakistan's tribal belt, said Islamabad was hemmed in by its US alliance and stands to lose more than it would gain by ending its cooperation with the war in Afghanistan.

"Geostrategic compulsions arising out of partnership with the United States and other allies basically restrict Pakistan from taking a public position on the drone strikes," he told AFP.

The review of the alliance will not result in any "earth-shattering recommendations", he said.