A senior Iranian lawmaker has warned Russia that continuing to delay the delivery of a missile-defense system to Iran will harm relations between the two countries. The comments were made as Russia's Deputy Foreign Minister Sergei Ryabkov was scheduled to visit Tehran.

The head of the Iranian parliament's national security committee, Alaedin Borujurdi, is warning Russia that any further delay in fulfilling a contract to sell an anti-aircraft missile-defense system to Tehran would "seriously damage" relations between the countries.

A Russian contract to sell sophisticated S-300 surface-to-air missiles to Iran has yet to be fulfilled, amid behind the scenes bickering between both countries.  The United States and other Western nations strongly oppose the sale of advanced Russian missiles to Tehran.

But Borujurdi, says a Western-backed nuclear deal to supply Tehran with highly enriched uranium is still on the table. He indicates Tehran wants to change a key provision in the deal calling for Iran to ship 80 percent of its low-enriched uranium abroad immediately, in return for a supply of more highly enriched uranium, later.

He offered a "limited exchange" deal in which Tehran would receive quantities of highly enriched uranium at the outset in exchange for a less substantial portion of its low-grade uranium.

Meanwhile, al Arabiya TV reports Russian Deputy Foreign Minister Sergei Ryabkov is visiting Tehran and met Said Jalili, the head of Iran's National Security Council.

Ryabkov has taken a part in nuclear negotiations between Tehran and the West, repeatedly urging patience with the Islamic Republic.

Saturday, Russian President Dimitri Medvedev told the German Der Spiegel magazine he would prefer to avoid the prospects of imposing further sanctions on Tehran, but that the prospect could not be excluded if nuclear talks fail.

Meir Javedanfar of the MEEPAS center in Tel Aviv says Russia has a great deal to lose if economic sanctions are imposed on Tehran and that it may be trying to avoid that prospect. "The Russian government is concerned that the recent deal offered to Iran is falling apart, because statements coming from Iran, which suggest that the fuel has to be shipped in smaller batches, plays contrary to the spirit of that agreement and as far as many countries of the West are concerned, it is a deal-breaker.  And should that finally happen and the whole deal falls through, Russia would be the country who could lose the most, because it would be put in the very difficult position where it would have to chose between its business interests with Iran and its relations with the West," he said.

Russia is in the process of helping Tehran build a much delayed civilian nuclear power plant at Bushehr.