Moscow's decision to halt delivery of powerful S-300 air-defense missiles to Iran, citing the new U.N. sanctions on the Islamic Republic, may be politically convenient but it could carry a hefty cost for Russia in the Middle East.

The Kremlin has been delaying delivery of five S-300PMU batteries under a $700 million contract in 2007, largely because of U.S. and Israeli pressure.

But Russian leaders have now found a valid excuse to deny the Islamic Republic the weapons it wants to protect its nuclear facilities from attack.

That may have earned Moscow brownie points in Washington but there is no doubt a quid pro quo involved somewhere in going along with the sanctions proposed by the Americans and their European allies.

Just what that might be isn't clear but there could well be a downside for the Russians and the arms industry they are striving to bolster as a means of restoring some of Moscow's influence in the Middle East as U.S. power is seen to be ebbing.

Iran will find it difficult to replace Russia as an arms supplier but it's likely to retaliate one way or another.

The Arab states, including those who were Cold War clients of the Soviets, have learned over the years that Moscow isn't the most reliable of arms suppliers.

Syria, Algeria, Egypt, Yemen and Libya fell into that category at various times.

Apart from the fact that its weapons systems have long been considered inferior to those produced by the Americans and to a lesser extent the British, French and Italians, deliveries have often been erratic.

Iran has become a major importer of Russian arms, due in large part to the U.S.-led weapons embargo the Islamic Republic has endured for the last 30 years.

This has given the Russians considerable clout with Tehran and Moscow has been able to use this to its advantage in its dealings with the United States.

But, as Jane's Defense Weekly observed recently in an analysis of Middle East arms procurement programs, the Arabs have a deep suspicion of Russian arms. That is likely to intensify now that Moscow is seen to be backing away from a major and controversial contract.

"Technological advances notwithstanding," Jane's noted, "there is a deeply rooted shortage of confidence in Russian-made hardware among the gulf states."

"Russia is trying to restore some of its power in the Middle East but its capability is limited because of the doubts about Russian technology," Mustafa Alani, director of security and terrorism studies at the Gulf Research Center in Abu Dhabi, noted in 2009.

"The lack of interest in Russian weaponry dates back to the 1950s and 1960s when Middle Eastern countries such as Egypt, Iraq and Syria pitted their Russian-made systems against Israel and its U.S.- and Israel-made weapons and found these systems wanting."

Russian has sought to restore its once-formidable defense industry and has had some success in selling aircraft, helicopters, armored vehicles and other systems to Algeria, Syria and Yemen.

But Egypt, which broke with Moscow in the early 1970s after the Soviets refused to sell it advanced jets and armor, and Iraq, a key buyer until Saddam Hussein was toppled in 2003, are no longer clients.

Efforts to muscle into the oil-rich gulf region, traditionally the preserve of U.S., British and French arms manufacturers, have so far not produced significant results.

The Saudis have dangled the prospect of arms buys worth up to $2.5 billion if Moscow scrapped the S-300 deal with Iran. Now that the missile deal seems to have been torpedoed, Riyadh may have to put its money where its mouth is.

But the Saudis probably have drawn a lesson from the March 2006 sale of 32 MiG-29 interceptors to Algeria in a $1.3 billion contract by Russia's state arms exporter, Rosoboronexport.

Algeria later refused to accept 24 of the aircraft because of their "inferior quality." Russia's defense ministry paid $690 million for the rejects after Algiers froze all payments on contracts with Russia in October 2007.

The MiGs were eventually returned to Russia in April 2008 and after being overhauled were assigned to Russia's air force.

Criminal charges were later made against executives of the Aviaremsnab aviation company that provided the MiGs sold to Algeria allegedly using forged airworthiness certificates.