A senior Iranian lawmaker warned Russia that its delay in delivering an anti-aircraft missile defense system to Tehran could harm relations between the two countries, state television reported Sunday.

Russia signed a contract two years ago to sell the S-300 surface-to-air missiles to Iran but has not made any deliveries or given any explanation for the delay. The United States and Israel strongly oppose the deal because it would significantly boost Iran's air defense capabilities at a time of heightened tension over the country's nuclear program.

Russia and the U.S. are among six nations leading an effort to ensure Iran does not use what it maintains is a civilian nuclear program to develop an atomic bomb. But Moscow also has close ties with Iran and is helping build its first nuclear power plant, forcing Russia into a delicate balancing act.

Russia's president warned over the weekend that Moscow could back sanctions against Iran if it fails to take a constructive stance in the nuclear talks.

On Sunday, a prominent Iranian lawmaker had a warning of his own over the delay in the missile system.

"If they do not fulfill their promise, this will be a negative point in relations between the two countries," said Alaeddin Boroujerdi, the head of parliament's national security and foreign policy committee, according to state TV.

The news report quoted Boroujerdi as saying that more delays "will be harmful to Russia since we have many areas of cooperation with them."

The S-300 surface-to-air missiles are capable of shooting down aircraft, cruise missiles and ballistic missile warheads at ranges of over 90 miles (145 kilometers) and at altitudes of about 90,000 feet.

In 2007, Russia delivered Iran another anti-aircraft system called the Tor-M1, which can hit aerial targets at up to 20,000 feet.

Russia is helping Iran build a nuclear power plant in the southern Iranian port of Bushehr whose launch has also been delayed and is now expected within several months.

In the international talks over Iran's nuclear work, a U.N.-brokered plan worked out in October requires Tehran to send 1.2 tons (1,100 kilograms) of low-enriched uranium _ around 70 percent of its stockpile _ to Russia by the end of the year for further enrichment.

That would help ease international concerns that the material could be further processed to make it usable for a bomb. Iran, which says its nuclear work is only for peaceful purposes like energy production, has not yet announced a decision on whether to accept the deal.