Russia may not deliver the S-300 air defence systems to Iran, according to Russian news agency Interfax. A deal for the system was signed by the two states in 2005. 
The unofficial word on the missile system came from Moscow.
An unnamed source in Moscow was quoted as saying that the possibility of Russia canceling the deal should not be ruled out. "The decision on this matter must be taken at the political level, because this contract is now well beyond the purely commercial framework," according to Interfax's source.
The S-300 long-range surface-to-air missiles could be deployed as a powerful defense against air raids on Iranian strategic sites. The system is capable of simultaneously tracking up to 100 targets, engaging 12 of them, at a range of up to 200 kilometers and a height of up to 27 kilometers.
The unofficial word on the missile system came from Moscow on the heels of a meeting between American and Russian officials last week, when they pledged to work towards agreed approaches to Iran, Afghanistan and North Korea.
In December 2008, Iran's state-run IRNA news agency reported that Russia had already begun delivering the S-300 systems. IRNA quoted Ismail Kosari, deputy head of Iran's Foreign Affairs and National Security committee. The Iranian Foreign Ministry declined to comment on the report.
In 2007, Russia supplied Iran with Tor-M1 short-range anti-aircraft missile systems. Iran reportedly deployed the Russian systems to guard its nuclear development sites from air attack.
Russia is still heavily involved in the Iranian nuclear program, especially the construction of the Bushehr nuclear power plant. US and other Western powers fear Iran is using the cover of nuclear power development to prepare a nuclear weapons arsenal.