Iran says it is developing its own missile defense system after Russia last year blocked delivery of the formidable S-300PMU system, which had been purchased in 2007.

Israeli defense expert Uzi Rubin, architect of the Jewish state's evolving multilayer missile defense program, says Tehran may be getting help from North Korean weapons engineers.

Iranian news agencies quoted defense officials as saying the Iranian Bavar 373 system is a substitute for the five S-300 batteries Moscow refused to deliver. The Iranians claim their system is more advanced than the Russian S-300, which was developed by NPO Almaz of Moscow.

"The designing phase of the Bavar 373 missile system … is to be completed soon," said Brig. Gen. Farzad Esmaili of the Islamic Revolutionary Guard Corps, which oversees missile development projects in Iran.

"We don't think about the S-300," he added. "This domestically built system has more advanced capabilities than the S-300."

"Intuitively, it's difficult to imagine that the Iranian system is as good as the S-300," Rubin told The Jerusalem Post. "Making the missile is the simple part. The problem is creating complex radars and other components.

"The effectiveness of the system depends on the radars. The Iranians have some skills in this but years of experience are needed. It's difficult to believe that this can be done in one generation."

However, Rubin observed, "there are indications they're not working alone."

He said North Korean engineers may be helping the Iranians, as they have frequently over the years. The Iranians, he noted, "may be on the way to reaching these capabilities."

Technology for North Korea's Taepodong series of ballistic missiles is widely considered to have been used in Iranian projects.

On May 13, a U.N. panel of experts submitted an 81-page report to the Security Council saying that Pyongyang, ever in need of funds, persistently exported, or attempted to export, ballistic missiles, missile components and the relevant technologies to Iran in recent years despite U.N. bans.

Russians, a key arms supplier to Iran in recent years, dragged its feet on the S-300s for more than a year before deciding that delivering the missiles was banned under the fourth round of U.N. sanctions imposed on Iran in June 2010 for refusing to abandon its purported nuclear arms program.

The new sanctions regime gave Moscow the cover to renege on the $800 million deal in September.

But the decision to withhold the S-300s was largely political and part of its effort to improve relations with the West to help modernize strategic economic sectors.

The S-300 issue became a secret test for U.S. diplomacy at the highest levels and provided an opening for some strategic horse-trading.

By several accounts, Israeli Prime Minister Binyamin Netanyahu agreed to sell the Russians 36 surveillance drones -- a field in which Russia is technologically deficient -- in a $100 million deal as part payment for scrapping the S-300 sale to Iran.

The United States and Israel had been particularly vehement in seeking to persuade Moscow to play ball because if Tehran acquired the S-300s its air-defense system would be dramatically strengthened to counter threatened pre-emptive Israeli airstrikes aimed at knocking out Iran's nuclear facilities.

The S-300 is considered one of the most advanced air-defense systems, ranking alongside the U.S. all-altitude Patriot system.

The S-300 can engage multiple targets, missiles as well as aircraft, at ranges of more than 100 miles at low and high altitudes.

Iran's air-defense system has nothing remotely as effective as the S-300.

The Russians are phasing it out with its own forces for the more advanced S-400. They're believed to be ahead of schedule in developing the S-500 system, which could be ready for production by the end of 2012.

In recent months, the Iranians have announced they're producing a range of indigenously developed weapons systems, including the Ghader anti-ship cruise missile.

There seems to be no shortage of new weapons programs, heightening Western skepticism about Tehran's endless claims.

While it's clear Iran has succeeded in making substantial technological advances, it's more likely to be focusing primarily on producing intermediate-range ballistic missiles like the Shehab-3 and the Sejjil-2 that are a crucial component of its nuclear program.

But air-defense is all-important as well, particularly as the Israelis are saber-rattling again and tension is mounting with the United States.