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."