The UN special investigator on counterterrorism has expressed his concern over civilian deaths from drone strikes in Yemen and Afghanistan, but says there has been a "significant de-escalation" in civilian casualties recorded in Pakistan.

UN special rapporteur Ben Emmerson presented his report to the UN Human Rights Council in Geneva on Wednesday, highlighting at least 59 civilian casualties in 19 drone strikes in 2013 in Afghanistan, a significant rise on the previous year.

In Yemen, he said: "The frequency of armed drone strikes appears to have intensified, particularly during the closing months of 2013, with a sharp escalation in the number of reported civilian casualties."

They included an attack in December, where at least 12 people believed to be part of a wedding convoy were killed.

"But or the first time in nine years there were no reports of civilian casualties during 2013 in the [tribal areas] of Pakistan," he said.

Emmerson said the US military had reduced attacks against armed groups, limiting strikes to high-value targets in response to the country's growing criticism of the programme. The total number of recorded strikes in 2013 was down to 27 from a peak of 128 in 2010, he said.

Pakistan says drone strikes hamper efforts to hold peace talks with the Taliban and breach national sovereignty.

However, rights group Amnesty International said that Emmerson's report may not be a definitive study of all drone strikes, and is based on the information that was available to him.

"Last year there were some reports that specific drone strikes resulted in civilian casualties [in Pakistan], but owing to the prevailing secrecy of the US programme and restrictions on access to these remote and lawless areas, it was impossible for us to investigate these claims further," said Mustafa Qadri, Amnesty's Pakistan researcher and co-author of the group's report into civilian casualties in the country.

He added that it was "impossible to give absolute answers because of legal issues of who may and may not be a 'combatant' or otherwise be lawfully targetable" without the US being fully transparent about their operations.

"How and when strikes happen, who is being targeted and why - this uncertainty will remain, and victims will not be able to get the justice they deserve," Qadri told Al Jazeera.

But Emmerson told Al Jazeera that his team "received no reports from any reliable source alleging civilian casualties during 2013 in Pakistan".

Emmerson recommended the council to adopt a resolution aimed at urging all states to "comply with their obligations under international law" and launching independent fact-finding inquiries into strikes in which civilians are reported to have been injured or killed.

Additional reporting by Al Jazeera's Rahul Radhakrishnan.