The second U.S. drone attack in as many days killed 10 people in northwest Pakistan on Sunday, intelligence officials said, an incident likely to raise tensions in the standoff between Washington and Islamabad over NATO supply routes to Afghanistan. The remotely-piloted aircraft fired four missiles in the Birmal area of the South Waziristan tribal region near the Afghanistan border, officials said. Another CIA drone strike in the same area killed two suspected fighters on Saturday. The United States and Pakistan are locked in negotiations to re-open overland supply routes to NATO forces in Afghanistan. Islamabad blocked the routes last November to protest the death of 24 Pakistani soldiers by cross-border friendly fire from NATO aircraft. The supply lines through Pakistan are considered vital to the planned withdrawal of most foreign combat troops from Afghanistan before the end of 2014.