UN Special Rapporteur Ben Emmerson, who issued the "interim" report, said on Thursday that the US had created "an almost insurmountable obstacle to transparency."

"The Special Rapporteur does not accept that considerations of national security justify withholding statistical and basic methodological data of this kind," wrote Emmerson in the report, which is due to be presented to the UN General Assembly next Friday.

US intelligence officials have consistently downplayed the number of civilian deaths from drone strikes. The UN report, however, revealed at least 400 in Pakistan and as many as 58 in Yemen were killed as a result of the US drone attacks.

In a major speech on drone strikes this May, President Barack Obama openly acknowledged civilian deaths, saying "they will haunt us for as long as we live" -- but didn't provide any hard numbers or estimates.

"It is a hard fact that US strikes have resulted in civilian casualties, a risk that exist in every war," Obama said. "And for the families of those civilians, no words or legal construct can justify their loss."

According to Emmerson states that Pakistani officials confirmed "at least 400 civilians had been killed as a result of remotely piloted aircraft strikes and a further 200 individuals [killed] were regarded as probably non-combatants."

Emmerson also said that he and his researchers had identified 33 "sample remotely piloted aircraft strikes that appear to have resulted in civilian casualties." Most of these were by the US, he said, but about "eight or nine" were Israeli strikes in Gaza. He did not identify the strikes, saying he is still investigating them and plans to present his findings to the UN Human Rights Council.

The highest level of civilian casualties, Emmerson said, occurred when the CIA ramped up drone strikes in Pakistan between 2008 and 2010.

Emmerson's estimate of civilian casualties from drone strikes in Yemen ranged from 21 to 58.

NTJ/BA