Russia's reputation will be hurt if it fails to fulfill a contract to deliver S-300 advanced air defence missiles to Iran, Tehran's ambassador to Moscow said Tuesday, quoted by Russian news agencies.

"Refusal to deliver the S-300s will strike a blow to Russia's reputation as a reliable arms supplier," Mahmoud Reza Sadjadi was quoted as saying.

"It would compel us to question Russia's reliability at such sensitive moments."

Iran's desire to acquire the S-300 surface-to-air missiles, which the West fears will help it guard against possible future air strikes, is at the heart of the crisis over the Iranian nuclear drive.

Russia has agreed the sale of the S-300s with Tehran but delayed delivery amid pressure from Israel and the United States -- neither of which have ruled out military force in the standoff over Iran's nuclear programme.

"We strongly hope the Russian government will fulfill its obligations," the Iranian diplomat said in Moscow. "It would be irrational for Russia not to abide by its commitments."

The comments came after Iranian President Mahmoud Ahmadinejad voiced disappointment in Russia, which has long enjoyed close ties with Tehran, for its backing of a new round of UN Security Council sanctions.

Western diplomats have said Russia would be unable to deliver the S-300 missiles under the new sanctions resolution being discussed.

But top Russian lawmakers have said the sanctions would have no effect on the S-300 sale as the contract has already been agreed.

The United States last week lifted sanctions against three Russian firms implicated in boosting Iran's nuclear programme, commending Moscow's "good example" in suspending supply of the S-300s.

The United States and its allies accuse Tehran of using its civilian nuclear drive as a cover for aims to build an atomic bomb, charges denied by the Islamic republic.

Russia, meanwhile, is helping Iran build its first nuclear power plant in the southern city of Bushehr which is expected to finally go online in August.