Iran says Russia is selling Tehran advanced anti-aircraft missiles, but did not say when they will arrive.
The S-300 air defense system has a radar that can track many targets at the same time, and missiles that can hit targets anywhere from just above the treetops to 27 kilometers in the sky.
The S-300 compliments a shorter-range Russian system sold to Iran earlier.
Iran's announcement that it is upgrading its air defenses was made during a time of international tensions over Iranian nuclear programs that western nations fear could be used to make nuclear weapons.
The United States has never ruled out military action against the suspected nuclear weapons projects, but a recently published U.S. intelligence report said Iran stopped its nuclear weapons program several years ago.
Some information for this report was provided by AFP and Reuters.