Noted human rights organisation Amnesty International has said the US may be guilty of war crimes over its use of drone strikes after finding evidence of civilian deaths, mainly in Pakistan and Afghanistan.
Amnesty said on Monday that it reviewed nine recent drone strikes in North Waziristan and found a number of victims were unarmed.
As a consequence, human rights campaigners are gearing up today to demand that American officials be held responsible for illegal killings carried out by drones and call for greater transparency over the secret programme conducted by the Central Investigation Agency (CIA).
A report by Amnesty details how civilians have been killed in Pakistan – including a 68-year-old grandmother who died in her family's fields. It comes amid intense scrutiny of the CIA's covert drone programme.
Nawaz Sharif, the Pakistani prime minister, is in Washington where he is expected to raise the issue of drones with Barack Obama days before a United Nations debate on the subject.
The strikes are intensely controversial in Pakistan, where they are frequently blamed for killing civilians and driving young men to terrorism.
In its report, Amnesty also asked the UK not to share intelligence, facilities or specialist components that might be used in strikes.
Mustafa Qadri, the report's author, said, ''Secrecy surrounding the drones programme gives the US administration a license to kill beyond the reach of the courts or basic standards of international law ... what hope for redress can there be for victims of drone attacks and their families when the USA won't even acknowledge its responsibility for particular strikes?''
In a separate report looking at six US attacks in Yemen, Human Rights Watch says two of them killed civilians at random, violating international law.
Drone warfare has become common in the US pursuit of al-Qaeda and the Taliban elements; but few details are known about the covert US drone operation except those emerging from the ground and the victims.
Senior al-Qaeda and Taliban leaders have been killed in drone strikes in Pakistan, but civilians have also died.
Last week, a UN investigation found that US drone strikes had killed at least 400 civilians in Pakistan, far more than the US has ever acknowledged.
Amnesty reviewed all 45 known drone strikes that took place in North Waziristan in north-western Pakistan between January 2012 and August this year. Contrary to official claims that those killed were ''terrorists'', campaigners concluded that in a number of cases the victims were not involved in armed activity and posed no threat to life.
In July last year, researchers found that 18 labourers, including a 14-year-old boy, were killed in multiple strikes on a village close to the border with Afghanistan as they were about to eat an evening meal at the end of work.
In October 2012, Mamana Bibi was killed in a double strike - apparently by a Hellfire missile - as she picked vegetables in the family's fields while surrounded by her grandchildren.
UN special rapporteur Ben Emmerson accused the US of challenging international legal norms by advocating the use of lethal force outside war zones.
A controversial aspect of the US policy is that drone attacks are carried out not by the military but by the CIA.
Defence Secretary Leon Panetta has argued in favour of the policy, saying that the US will continue to defend itself.
President Barak Obama has insisted the strategy was "kept on a very tight leash" and that without the drones, the US would have had to resort to "more intrusive military action".