According to unconfirmed reports Iran has purchased 4 S-300 (NATO designation, SA-10 Grumble) surface to air missile platforms, possibly in an effort to prevent a U.S. or Israeli air strike.
Though currently denying the allegations, Belarus is said to have supplied two of missile platforms and the other two come from an another as yet unknown source.
The report could be a ploy by the Iranians to delay or avoid an air strike, but if they do have the missile systems an air strike will be more difficult. Those difficulties aside the U.S. is still said to have a viable attack plan for Iran.
Though Belarus is denying the sale, Janes Defence Weekly reported as far back as January 2008 that they were in negotiations to purchase the S-300's from them.