A U.S. drone aircraft killed eight suspected militants in North Waziristan on Sunday, security officials said.A drone missile struck a house in the Shawal Valley where militants were reported to be hiding in the North Waziristan tribal region near the Afghan border."Two missiles were fired on a house. Eight militants were killed," said a local intelligence official.Several of the men killed were loyal to Hafiz Gul Bahadur, a top militant leader in North Waziristan, the official said on condition of anonymity.During last week a similar attack in the region had killed five militants. Sixty-four US drone strikes were carried out in Pakistan's semi-autonomous tribal belt last year, according to AFP. The US claims the airstrikes target Taliban militants. But locals say civilians are the main victims.The London-based Bureau of Investigative Journalism has said that under President Barack Obama one drone strike has hit Pakistan on average every four days.It said most of the 2,292 to 2,863 people reported to have died were low-ranking militants, but that only 126 fighters had been named.It said it had credible reports of between 385 and 775 civilians being killed, including 164 to 168 children.UN High Commissioner for Human Rights Navi Pillay last month called for a UN investigation into US drone strikes in Pakistan, questioning their legality and saying they kill innocent civilians.The UN human rights chief provided no statistics but called for an investigation into civilian casualties, which she said were difficult to track.She said UN chief Ban Ki-moon had urged states to be "more transparent" about circumstances in which drones are used and take necessary precautions to ensure that the attacks involving drones comply with applicable international law.