The United Nations has said that at least 450 civilians may have been killed by US drone strikes in Pakistan, Afghanistan and Yemen, a figure which the US has previously downplayed.

A UN report, obtained by Al Jazeera on Friday, stated that Pakistan’s government had confirmed at least 400 civilian deaths as a result of US drone strikes, in stark contrast to what US officials had publicly acknowledged previously.

The report found that one of the major obstacles in obtaining accurate figures on civilian deaths was the lack of transparency by the countries involved, which has prompted a cauldron of legal issues that are yet to be addressed by UN-member states.

"This report aims to present the facts as clearly and objectively as possible. I have had good co-operation from most of the states involved, and I very much hope that this continues during the second phase of the inquiry when I will be aiming to get answers to some of the most difficult questions and putting allegations about particular drone strikes to the states responsible, and asking them to provide their own version of the facts," Ben Emmerson, the Special Rapporteur on Human Rights and Counter-Terrorism and author of the report, told Al Jazeera.

"This issue is clearly not going to go away, and I will continue asking the difficult questions for as long as it takes," he added.

As there were, or appear to be, reported cases of civilian deaths, the report states that the US has a legal obligation to launch its own impartial investigation and provide a public explanation because of its duty to protect civilians in an armed conflict.

The US, however, has not released any casualty figures so far from its CIA-led drone programme, especially in Pakistan and Yemen, but the spy agency has acknowledged that the figures were in “single digits”, according to media reports.

The report, which is to be presented to the UN General Assembly on October 25, recommends that the UN member-states implement strict compliance to international humanitarian law and identifies a number of legal questions on the on the usage of drones which currently has no international consensus.

Transparency

It also urged the US to be more transparent with its information on the drone campaign to ascertain the resultant civilian deaths.

"The single greatest obstacle to an evaluation of the civilian impact of drone strikes is lack of transparency, which makes it extremely difficult to assess claims of precision targeting objectively," wrote Emmerson.

The UN investigator, who heads the UN inquiry into the civilian impact of drone strikes launched earlier this year, considers the continued use of drones in the Federally Administered Tribal Areas amounts to be a violation of Pakistani sovereignty, "unless justified under the international law principle of self-defence", which enlists that a state can act upon armed groups, which may or may not have ties to another state, if it considers it a threat to its own national security.

He said that the use of lethal force under the pretext of counter-terrorism operations by the US outside of areas of active hostilities, "gives rise to a number of issues on which there is either no clear international consensus, or United States policy appears to challenge established norms."

White House spokesperson Laura Magnuson told NBC news that they were aware of the new UN report and cited Obama’s speech in May in which he defended his administrations' use of drones and narrowed the scope of the targeted-killing campaign against al-Qaeda and its affiliates.

The Washington Post newspaper, citing documents provided by whistleblower Edward Snowden, reported on Wednesday that the CIA-operated drone campaign relied heavily on NSA's ability to intercept communications, leading to strikes targeting al-Qaeda fighters abroad.

In June 2011, John Brennan, Obama's top counter-terrorism adviser and current CIA chief, said that were no collateral deaths "because of the exceptional proficiency, precision or the capabilities we've been able to develop".

Emmerson said that the involvement of the CIA in US operations in Pakistan and Yemen "created an almost insurmountable obstacle to transparency", reiterating an earlier UN call for increased transparency over armed drones.

In August, US Secretary of State John Kerry told Pakistan state TV that drone strikes in the country could end soon as the threat of insurgency recedes. Pakistani government records estimate at least 2,200 deaths in drone strikes since 2004.

The report also indicated that at least 50 civilians were also killed in strikes on Afghanistan and Yemen.

The Special Rapporteur intends to submit a final report on his findings from the inquiry to the Human Rights Council in 2014.