ISLAMABAD: A UN expert on Friday called on the US to reveal the number of civilians it believed had been killed by American drone strikes targeting militants.

UN Special Rapporteur Ben Emmerson said that information gathered for a new report indicated that more than 450 civilians might have been killed by drone strikes in Pakistan, Afghanistan and Yemen, but more work needed to be done to confirm the figures.

“The single greatest obstacle to an evaluation of the civilian impact of drone strikes is lack of transparency, which makes it difficult to assess claims of precision targeting objectively,” said the report.

The US and other countries that use drones have an obligation to investigate reported civilian casualties and clarify their legal justifications for the attacks, according to the report. The Pakistani government told the UN that at least 400 civilians had been killed by US drone strikes in the country --- a figure that Emmerson first revealed during a visit to Islamabad in March.

The report said the UN had confirmed that more than 30 civilians were killed in drone attacks in Afghanistan in 2012 and 2013. Media reports indicated that at least 21 civilians had been killed in strikes in Yemen since 2011.

In total, Emmerson has identified 33 drone attacks that appear to have resulted in civilian casualties, but he is still trying to confirm his findings. He will present his final report to the UN Human Rights Council once he is finished.—AP