American UAV strikes, most notably in Pakistan and Yemen, have shot up since Barrack Obama came to power. Estimates state that while there were 52 such strikes during George W Bush's time, this number has risen to 282 over the past three and a half years, with officials justifying it has international “self defence” against a stateless enemy.

Mr Emmerson said it was time for the US to open itself up to scrutiny as to the legality of such attacks. While it remains nigh on impossible for observers to establish the truth on the ground in many of areas, each strike is visually recorded and videos could be passed to independent assessors, he explained.

“We can't make a decision on whether it is lawful or unlawful if we do not have the data. The recommendation I have made is that users of targeted killing technology should be required to subject themselves, in the case of each and every death, to impartial investigation. If they do not establish a mechanism to do so, it will be my recommendation that the UN should put the mechanisms in place through the Human Rights Council, the General Assembly and the Office of the High Commissioner”, he said.

He continued: “The Obama administration continues formally to adopt the position that it will neither confirm nor deny the existence of the drone program, whilst allowing senior officials to give public justifications of its supposed legality in personal lectures and interviews. In reality the administration is holding its finger in the dam of public accountability. There are now a large number of law suits, in different parts of the world, including in the UK, Pakistan and in the US itself, through which pressure for investigation and accountability is building.”

Recently Wajid Shamsui Hasan, Pakistani High Commissioner, said the US strikes “violated” his country and encouraged extremism while last month Navi Pillay, UN Commissioner on the Protection of Civilians in Armed Conflict said she was “seriously concerned” by reports of civilian deaths in Yemen, Pakistan and Somalia.