An advanced version of S-300, called S-300PMU1 (SA-20 Gargoyle), the missile system has a range of over 150 kilometers (about 100 miles) and can intercept ballistic missiles and aircraft at low and high altitudes, making this system an effective tool for warding off possible air strikes on Iran.
"S-300 air defense systems will be delivered to Iran under an earlier contract signed with Russia," the Fars news agency quoted Iranian Defense Minister Mostafa Mohammad Najjar as saying.
The minister provided no contract details or a schedule for future deliveries, Fars said.
The signing of the deal follows last week's session of the Russian-Iranian commission on military-technical cooperation in Tehran, where the sides reviewed existing agreements and discussed future steps to extend cooperation in the military sphere.
The closest western equivalent of S-300 is the U.S. MIM-104 Patriot system or the U.S. Navy RIM-66 Standard Missile 2 (SM-2).
U.S. authorities have repeatedly called on Russia to stop arms deliveries to countries whose political regimes Washington disapproves of, including Iran.
However, Russian President Vladimir Putin said in October that Russia would not take into consideration attempts to impose arms deal restrictions "based on unilateral and politicized assessments".
He also said deliveries of Russian weapons were aimed exclusively at increasing the defense capability of the countries receiving them, and at maintaining their stability.
Russia earlier supplied Iran with 29 Tor-M1 air defense systems under a $700-million contract signed in late 2005.
In February, Tehran successfully tested Tor-M1s during a military exercise by the Islamic Revolutionary Guards Corps in southern Iran.
The Iranian defense minister said on Wednesday that during last week's meeting Russian and Iranian officials had discussed the possibility of using Russian experts to train the crews of Iranian Tor-M1 systems.
