Irrespective of Kremlin denials, Iran is buying the Russian-built SA-20 strategic-range air defense system, say senior U.S. government officials.

Deployment of the system - a threat previously thought to be only a bargaining tool - would mark a capability leap in the Middle East and considerably improve Iran's ability to defend its nuclear facilities. Western officials are concerned that Tehran is pursuing nuclear weapons.

"The Iranians are on contract for the SA-20 [which NATO designated Gargoyle']," says one of the U.S. government officials. "We've got a huge set of challenges in the future that we've never had [before]. We've been lulled into a false sense of security because our operations over the last 20 years involved complete air dominance and we've been free to operate in all domains."

The proliferation of so-called double-digit surface-to-air missile systems - such as the Almaz Antey SA-20 (S-300PMU1/S-300PMU2) - poses an increasing threat to nonstealthy aircraft, and will force changes in tactics and operational planning. The SA-20 has an engagement envelope of roughly 100 mi., and Iran may be signed up for the S-300PMU-2 variant with even greater range.

Russia could use Belarus as the route for a sale, allowing Moscow to deny any direct involvement, says a U.S. official. It would likely take the Iranian armed forces as long as 24 months to become proficient in the operation of the SA-20; however, any deal would almost certainly cover training support in the interim. Israel might be tempted to preemptively strike suspected nuclear sites prior to the SA-20 becoming operational, or even try to hamper delivery.

"The beginning of proliferation of double-digit SAMs is more of a concern than the potential air threats [such as Russia's Sukhoi Su-35 and China's Chengdu J-10] that are coming into service," says the government official.

The presence of Russian double-digit long-range SAM systems in the region during the recent Georgian incursion had a direct impact on NATO planning - resulting in a decision not to use the Boeing E-3 AWACS for surveillance.

The SA-20 and, even more so, the SA-21 Growler (S-400) now entering service pose an increasing problem for mission planners using conventional strike aircraft. While low-observable aircraft offer greater latitude for operations, they are not totally immune to air defenses.

The Lockheed Martin F-22 with its all-aspect, -40-dBsm. radar cross-section signature can operate within the engagement envelope of the SA-20 and SA-21. But the Lockheed Martin F-35 with its -30-dBsm. signature, but not all-aspect stealth, is at greater risk. The rear quadrant of the F-35, particularly around the engine-exhaust area, is not as stealthy as the F-22.

Because of its aging stealth design, the Northrop Grumman B-2 also has limitations in the amount of time it can spend within the range of double-digit SAM systems, since small signature clues can become cumulative and offer a firing solution. The U.S.'s next-generation bomber program is aimed at developing a low-observable platform capable of operating irrespective of the threat from systems of the SA-21 class.