FILE PHOTO: Moscow - Russia - 07/05/2017 - Russian army S-400 Triumph medium-range and long-range surface-to-air missile system rehearse before the World War II anniversary in Moscow. REUTERS/Maxim Shemetov

MOSCOW (Reuters) - Russia has started delivering its S-400 surface-to-air missile system to China under the terms of a contract signed in 2014, the TASS news agency cited an unnamed source in the Russian military-industrial complex as saying on Thursday.