His comments came as Pakistani officials said that a US drone strike had killed at least four militants after targeting their vehicles in North Waziristan on Sunday. Attacks by American unmanned aerial vehicles (UAVs) are deeply unpopular in the country, which claims they violate its sovereignty and fan anti-US sentiment.

Only last week cricketer-turned-politician Imran Khan vowed to defy Taliban threats to attend a rally in Pakistan's tribal areas aimed at highlighting the human cost of US drone strikes.

Mr Emmerson, a leading London barrister and UN Special Rapporteur on Human Rights and Counter-Terrorism, said America is facing mounting global pressure over its use of UAVs and he is preparing a report for the next session of the Human Rights Council in March. The issue, he insists, will “remain at the top of the UN political agenda until some consensus and transparency has been achieved”.

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.

Mr Khan, who heads the Pakistan Movement for Justice party (PTI) intends to join a rally in Miranshah, North Waziristan, next month to protest against the US's policy of using drones to target suspected militants, when civilians get caught in the crossfire.

“During the last session of the UN Human Rights Council in Geneva in June many states, including Russia, China and Pakistan called for an investigation into the use of drone strikes as a means of targeted killing. I was asked by these states to bring forward proposals on this issue and I am working closely on the subject of drones with Christof Heyns the UN Special Rapporteur on extrajudicial, summary and arbitrary execution. The issue is moving rapidly up the international agenda,” explained Mr Emmerson, who has called for the “end to the conspiracy of silence”.

Last month, the Center for Constitutional Rights (CCR) and the American Civil Liberties Union filed a law suit against senior CIA and military officials over the killing of three Yemeni-American citizens, including a 16-year-old boy. The first hit in September 2011 killed the extremist imam Anwar Al-Aulaqi and alleged al-Qa'ida propagandist Samir Khan.

“Two weeks later, on October 14, US drone strikes killed Anwar Al-Aulaqi's son, 16-year-old Abdulrahman Al-Aulaqi as he was eating dinner with his teenage cousin at an open-air restaurant,” said the CCR, claiming that the “escalated and expanded” use of drone strikes since the Obama administration came to power had killed hundreds of civilians amongst the estimated 2,500 deaths.

In May, White House counter-terrorism advisor John Brennan defended the targeted killings of al-Qa'ida suspects as effective tool to minimise civilian casualties, likening the use of drones to laser surgery eliminating “the cancerous tumour called an al-Qa'ida terrorist”.

A key aspect of any investigation would be whether strikes are taking place within a theatre of war such as Afghanistan or on sovereign territories outside the conflict zone despite US assertions that they are conducting a global war against a stateless enemy.

More than 50 countries worldwide now have the use of UAVs. The most dynamic sector in the aviation industry is estimated to be worth £4 billion a year with millions being poured into researching more sophisticated varieties such as autonomous drones that do not even require remote pilots.

The RAF's Predator MQ-9 Reapers, launched from Kandahar but piloted from Creech Air Force Base in Nevada by 39 Squadron, are used for surveillance in Afghanistan.

MoD figures from May state that UK drones had flown a total of 34,750 hours and fired 281 missiles and laser-guided bombs.

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