This page contains a listing of major foreign holders of U.S. Treasury securities (in billions of dollars). "These estimates of monthly holdings of Treasury securities by country are derived from a combination of monthly data on foreign holdings of short-term Treasury bills and certificates and periodic surveys of holdings of long-term Treasury bonds and notes that are updated using monthly data on foreign net purchases of long-term Treasury securities. An additional adjustment is made to include monthly holdings of non-marketable Treasury securities."
United States. Dept. of the Treasury: http://www.treas.gov/tic/