The USA must disclose details of the legal and factual basis for the lethal use of drones in Pakistan, Amnesty International said today, after US President Barack Obama confirmed that the unmanned aircraft are used to target suspected militants in the country's tribal areas.

President Obama made the rare public acknowledgment on Monday during an hour-long online video chat with users of the social network Google+.

Amnesty International also called for the USA to monitor civilian casualties inflicted by drone attacks in Pakistan.

Sam Zarifi Amnesty International’s Asia-Pacific director, said:

“The US authorities must give a detailed explanation of how these strikes are lawful and what is being done to monitor civilian casualties and ensure proper accountability.

"What are the rules of engagement? What proper legal justification exists for these attacks? While the President's confirmation of the use of drones in Pakistan is a welcome first step towards transparency, these and other questions need to be answered.”

President Obama said that the drone strikes, which are carried out by the CIA rather than the military, were a “targeted focused effort at people who are on a list of active terrorists”.

He asserted that the strikes targeted “al-Qaeda suspects who are up in very tough terrain along the border between Afghanistan and Pakistan”.

US drone attacks have doubled overall in Pakistan during the Obama administration. Thousands of people have been killed by the strikes - civilians as well as militants.

Because of the security situation and difficulty in accessing the terrain it has been impossible for organisations like Amnesty International to verify the number of civilian casualties caused by drones.

Available evidence shows that the number of strikes decreased during 2011.

In its June 2010 report As if Hell Fell on Me: The Human Rights Crisis in Northwest Pakistan, Amnesty International said the use of drones to target insurgents in northwest Pakistan had generated considerable resentment inside the country.

US Attorney General Eric Holder will reportedly reference US legal arguments in support of such killings by drone attacks in a speech on national security in the coming weeks.

Sam Zarifi added:

“The US administration must use the occasion of Attorney General Holder's speech to disclose the relevant legal and factual documentation necessary for a meaningful assessment of the lawfulness of the deliberate killings it is carrying out - simply trying to find another way to say 'trust us, it's legal' will not be good enough.”

Past justifications offered by US officials have invoked legal theories based on a “global war” between the USA and al-Qa’ida, a concept which is not recognised by international humanitarian or human rights law.