"Disclosure of these killings is critical to ensure accountability, justice and reparation for victims or their families," he said in a 28-page report.
“The (US) government should clarify the procedures in place to ensure that any targeted killing complies with international humanitarian law and human rights and indicate the measures or strategies applied to prevent casualties, as well as the measures in place to provide prompt, thorough, effective and independent public investigation of alleged violations,” he said.
The report also refers to an alarming increase in the use of drones over the past three years.
Thousands of people have been killed in 300 drone strikes in Pakistan since 2004, the report said, adding that at least 957 Pakistanis lost their lives in such attacks in 2010 alone.
"Although figures vary widely with regard to drone attack estimates, all studies concur on one important point: there has been a dramatic increase in their use over the past three years," Heyns said.
The US has conducted numerous strikes in Afghanistan, Iraq, Pakistan, Somalia and Yemen.
Washington claims the drones are only targeting militants but fatalities and causalities from such strikes have mostly been civilians.
A UN investigator has condemned the use of deadly drones by the United States, questioning the legality of such attacks that have led to the death of thousands of people.
Christof Heyns, a UN special rapporteur on extrajudicial summary or arbitrary executions, said that the US should clarify its policy of drone assassinations and targeted killings.
In a report to the UN Human Rights Council, the UN official urged the Obama Administration to abide by international laws and ensure accountability and justice for victims of the attacks.