Another lawyer argued before the US House Subcommittee on National Security and Foreign Affairs that while the United States had the right to use the drones, the CIA personnel actually launching the attacks could be guilty of war crimes.

A third expert tried to draw parallels between the killings in Fata and targeted killings of bandits loyal to Pancho Villa along the Mexican border in the 19th century. He insisted that current intelligence laws “implicitly” gave the US president the power to launch targeted killings.

Lawyers and lawmakers both noted that what was a seldom-used tactic in the Bush Administration, the use of Predator drones to launch attacks against Pakistani territory has become ubiquitous since President Obama took office last year.

In the eight years of George W. Bush's presidency, unmanned aircraft — or drones — attacked militant targets 45 times.

Since President Obama took office, the numbers have risen sharply 51 last year and 29 so far this year.

Most attacks have targeted suspected militant hideouts in Pakistan.

While the experts were divided over the legality of the drone attacks, America's human rights group, the American Civil Liberties Union, was not.

Based on reports about the scope of the programme, the American Civil Liberties Union took the position that the administration's programme of targeted killing outside of armed conflict zones was unlawful.

The group sent a public letter to President Barack Obama on Wednesday that said the drone attacks were part of an illegal programme authorised by the administration allowing suspected terrorists — including Americans — to be targeted and killed by US operatives.

Congressman John Tierney, the subcommittee's chairman, said that “the United States is committed to following international legal standards. Our interpretation of how these standards apply to the use of unmanned weapons systems will set an example for other nations to follow”.