Riyadh has reportedly been mulling the purchase of S-300PMU Russian air-defense missile systems and other arms worth as much as $4 billion, possibly as an inducement to Moscow not to supply such advanced weapons to Iran.

The Russians are doing just that and their relations with Tehran appear to going sour. So what's holding up a deal with Riyadh that would signal a potentially major geopolitical shift in the Middle East?

According to Mark N. Katz, professor of government and politics at George Mason University in Virginia, "Russia's relationship may not be the most important factor in Saudi-Russian arms negotiations and the delay in reaching a deal on air-defense missile systems in particular may be because of other reasons."

Russia has refused to deliver five S-300 batteries to Tehran under a 2007 contract, citing "technical difficulties." But this is almost certainly the result of intense pressure from the United States, Israel and Saudi Arabia.

This has incensed Tehran, which desperately wants the S-300s to protect Iran's nuclear sites against Israeli, or U.S., attack. On March 8, it ordered all Russian commercial pilots operating in Iran to leave within two months.

The S-300 can shoot down missiles and aircraft from a range of 100 miles and tackle a dozen targets simultaneously, thus making any airstrike a costly undertaking.

Further, possibly indefinite, delays in delivering the S-300s to Iran are likely, particularly since Moscow has given the nod to increased U.N. sanctions against Iran to pressure it into abandoning its alleged quest for nuclear weapons.

"Russian frustration with Iran has grown so strong that Moscow hardly needs to be persuaded by Riyadh or anyone else to withhold delivery of the S-300s to such a difficult customer," Katz argued in an analysis published on March 18 by The Moscow Times.

The mouthwatering Saudi arms deal is just what Russia wants to bolster its sagging arms industry by boosting its share of the lucrative Gulf market, which has traditionally bought Western arms.

The deal followed a decision by King Abdallah of Saudi Arabia in 2007 to cultivate Moscow as a counterweight to the United States in the aftermath of 9/11.

The deal was apparently set in motion with Russia's state-owned arms exporter, Rosoboronexport, in February when Vladimir Putin, who was Russia's president at the time, made a landmark visit to Riyadh.

According to various sources, the deal involves 12-18 S-300 batteries, as many as 30 Mi-35 assault helicopters and up to 120 Mi-17 transports, 150 T-90 main battle tanks and 250 BMP-3 infantry fighting vehicles.

So what's delaying the Saudi purchase? According to Katz: "One theory is that Riyadh does not want to buy the S-300 but the newer and more powerful S-400 instead …

"But while the Russian defense industry would be happy to sell the S-400 as well as other modern weapons to Saudi Arabia, Russia's air-defense force wants to acquire all S-400s produced in the next few years for its own long-neglected needs."

Russia recently activated its first S-400 battalion, with eight launchers, each with four missiles, around Moscow. Two more battalions are expected to deploy there by the end of this year.

The S-400 has a range of 250 miles and includes systems to counter electronic countermeasures used by U.S. forces. Moscow reportedly plans to acquire 200 launchers by 2015 and phase out the older S-200 and S-300 systems that are currently operational.

Katz noted that some Russian sources "also see pressure from Washington as a reason why Riyadh hasn't signed a major arms agreement with Moscow.

"But this is unlikely. If anything, the ongoing tensions in U.S.-Saudi relations -- as well as the Saudi desire to be seen as independent from Washington -- would push Riyadh more in the direction of buying weapons from Russia."

Thus, he concluded, "it seems that a major Saudi weapons purchase from Russia is highly likely. It is just the timing that is in doubt."

If the deal does ahead, Saudi Arabia would be the first foreign customer of the S-400. Moscow would thus become a direct threat to the U.S. arms industry.

Major missile manufacturers like Lockheed Martin and Raytheon have been trying for years to persuade Saudi Arabia and the other members of the Gulf Cooperation Council -- the United Arab Emirates, Kuwait, Qatar, Oman and Bahrain -- to buy interoperable U.S. Patriot PAC3 batteries.