[...]

The UN has already criticized the US drone program in the October interim report “on the use of remotely piloted aircraft in counter-terrorism operations,” authored by Special Rapporteur Ben Emmerson.

The report, the final version of which is due in 2014, argues the number of civilians killed in anti-terrorist drone operations is higher than publicly acknowledged, and condemns the US for lack of transparency over the issue.

"The Special Rapporteur does not accept that considerations of national security justify withholding statistical and basic methodological data of this kind," Emmerson wrote in the report.

Also in October, Human Rights Watch and Amnesty International simultaneously released reports on the use of drones in Yemen and Pakistan, both questioning the legitimacy of the strikes.

“Amnesty International is seriously concerned that these and other strikes have resulted in unlawful killings that may constitute extrajudicial executions or war crimes,” the AI report reads.

“President Obama says the US is doing its utmost to protect civilians from harm in these strikes. Yet in the six cases we examined, at least two were a clear violation of the laws of war,” said Humans Rights Watch Senior Researcher, Letta Tayler.

The US then reacted by saying it did nothing illegal.

"To the extent these reports claim that the US has acted contrary to international law, we would strongly disagree,” said White House spokesman, Jay Carney. "The administration has repeatedly emphasized the extraordinary care that we take to make sure counterterrorism actions are in accordance with all applicable law."

The United States has stepped up drone strikes in Yemen, as part of its crackdown on Al-Qaeda in the Arabian Peninsula (AQAP), believed to be the terrorist network’s main stronghold.