"The delay is due to technical problems. The delivery will be carried out when they are resolved," Alexander Fomin, deputy head of Russia's Federal Service for Military-Technical Cooperation, was quoted as saying.
Russia's contract to sell the S-300s to Iran has raised hackles in the United States and Israel, which believe that Tehran could use the sophisticated air defence missiles to defend its nuclear facilities against attack.
Western powers suspect that Iran is seeking to build an atomic bomb under the guise of its civilian nuclear energy programme, although Tehran says the programme is peaceful in nature.
Neither the United States nor Israel have ruled out air strikes in order to prevent Iran from acquiring nuclear weapons. Analysts say that S-300s could greatly complicate such air strikes.
Fomin, whose service oversees Russian arms exports, made the comments in an interview with Interfax while attending a defence exhibition in New Delhi, DefExpo India 2010.
He did not clarify what the technical problems were or how long it would take to fix them, Interfax reported.
Russia has been secretive about the Iran missile contract, but Interfax has reported that it calls for Moscow to sell Tehran five batteries of S-300PMU1 missiles for around 800 million dollars (530 million euros).
The S-300PMU1 — codenamed the SA-20 Gargoyle by NATO — is a mobile system designed to shoot down aircraft and cruise missiles.
Iran has expressed frustration with the delay in the missile delivery, and last week a top Iranian military commander said Tehran would build its own air defence missiles that would be even better than the S-300s.
Fomin's comments about the delay come at a time when Russia's once-warm relations with Iran have become strained amid the international dispute over Tehran's nuclear programme.
On Tuesday, Russia joined the United States and France in criticising a new push by Iran to step up uranium enrichment, and Moscow said it could not exclude a new round of sanctions against the Islamic Republic.
The prime minister of Iran's arch-foe Israel, Benjamin Netanyahu, visited Moscow this week in a bid to get Kremlin support for new sanctions.
Moscow has long had close ties with Tehran and is building Iran's first nuclear power plant in the southern Iranian city of Bushehr.
