WASHINGTON - The U.S. State Department said on Thursday it was "deeply, deeply troubled" by new reports of mass graves in Myanmar's Rakhine State, where the military has been accused of atrocities against minority Rohingya Muslims.The Associated Press reported earlier it had confirmed the existence of more than five previously unreported mass graves in the Myanmar village of Gu Dar Pyin, through interviews of survivors in refugee camps in Bangladesh and through time-stamped cellphone videos."We are deeply, deeply troubled by those reports of mass graves," State Department spokeswoman Heather Nauert told a regular news briefing. "We are watching this very carefully. We remain focused on helping to ensure the accountability for those responsible for human rights abuses and violations."Nauert said the reports highlighted the need for authorities in Myanmar to cooperate with an independent, credible investigation into allegations of atrocities in northern Rakhine state.