Former U.S. Ambassador to Russia William Burns assumed on Friday the post of the U.S. Deputy Secretary of State. \"Today, Bill Burns began serving as the Department of State\'s newest Deputy Secretary of State... I am grateful for Bill\'s decision to continue his nearly 30 years of service to the American people as we implement President Obama\'s ambitious foreign policy agenda,\" U.S. Secretary of State Hillary Clinton said in a statement. \"He has been on the frontlines during some of the most significant foreign policy breakthroughs in recent years, from building international consensus on free trade, to curbing the nuclear threat posed by Iran, to nurturing democracy in the Middle East, to helping negotiate the historic START arms control treaty with Russia,\" she added. Burns, 55, served as the U.S. ambassador to Russia between 2005 and 2008 and after his return to Washington worked as U.S. undersecretary of state for political affairs.