Former U.S. spy agency contractor Edward Snowden's whereabouts were a mystery on Monday as Russia defied White House pressure to send him back to the United States and stop him fleeing Moscow on his globe-crossing escape from U.S. prosecution. The 30-year-old flew to Moscow as a transit stop before heading elsewhere, several sources said. But reports that he would fly to Cuba were put in doubt when witnesses could not see him on the plane, despite heightened security before take-off.