A United Nations investigation has found that American drone strikes have killed at least 400 civilians in Pakistan, far more than the US has acknowledged.
Ben Emmerson, UN special rapporteur, accused the US of challenging international legal norms by advocating the use of lethal force outside war zones.
The investigation was launched in January and had been due to be completed by the autumn, offering a review of the legality of drone strikes and an analysis of the number of civilian casualties – totalling almost 500 across the world.
However, Mr Emmerson said gathering direct data on individual drone strikes – generally occurring in remote, hostile locations – had taken longer than expected and his report would not be complete until next year.
Instead, he has issued an interim report, which is due to be debated at the UN next week.
American officials have consistently played down the number of civilian casualties, insisting that precision targeting meant the annual number was in the "single digits".
In his report, Mr Emmerson cited Pakistani government statistics showing that 2,200 people had died in drone strikes in the past decade in Pakistan, of which at least 400 were civilians.
"Officials indicated that, owing to under-reporting and obstacles to effective investigation, those figures were likely to be an underestimate," he wrote.
Figures have only recently been made available in Afghanistan, where the UN says 31 civilians have been killed in drone strikes in the past 18 months.
Drone attacks are a source of widespread anger in Pakistan, where the government publicly condemns the missiles strikes as an assault on its sovereignty.
Yet there is evidence that politicians and military commanders have privately given the green light to missions.
Supporters insist the aircraft are responsible for cutting a swathe through the senior leadership of al-Qa'ida and other terrorist groups in Pakistan.
Mr Emmerson, a British human-rights lawyer, also criticised the US for failing to release its own casualty data and called for greater clarification of the legal justification for the strikes. (© Daily Telegraph, London)