The Associated Press
Russian officials have consistently denied claims that it already has provided some of the powerful missiles to Iran and had not clarified whether a contract existed.
The state-run ITAR-Tass and RIA-Novosti news agencies and the independent Interfax quoted an unnamed top official in the Federal Military-Technical Cooperation Service as saying Wednesday the contract had been signed two years ago.
Service spokesman Andrei Tarabrin told The Associated Press he could not immediately comment.
Supplying the S-300s to Iran would markedly change the military balance in the Middle East.