Ben Emmerson, a British lawyer and the United Nations special rapporteur on counter-terrorism and human rights, released a statement after a three-day trip to Islamabad Khaama Press reported Friday. He said officials told him there has been no "tacit consent by Pakistan to the use of drones on its territory."
A U.N. human rights team has been gathering information on the strikes.
"Adult males carrying out ordinary daily tasks were frequently the victims of such strikes," it said.
The Obama administration, which has increased the use of drones in Pakistan's tribal areas, has denied reports of hundreds of civilian casualties. The New America Foundation, however, estimates drones have killed between 1,953 and 3,279 people in Pakistan since 2004 -- about one-fifth of them civilians.
The foundation said the strikes have become more accurate, with 90 percent of the casualties in 2012 being militants.