Iran has a deal to buy S-300 anti-aircraft missiles from Russia. But Moscow has not delivered them yet.
The S-300 missile is a defensive weapon with a maximum range of about 200 kilometers.
"It's sort of similar to the American Patriot missile and it's bigger," explains Ivan Oerlich, with the Federation of American Scientists. "But it's specifically designed to be a broad-range, long-range defense system. It's not for close-in defense, but it's designed for aircraft, whether traveling at very high altitude or very low altitude, and to shoot down airplanes and cruise missiles. It's mobile. They can launch them off the back of a large truck and they carry several missiles. It means that it doesn't have a fixed location, and that makes it harder to destroy on the ground before it can be used."
Matthew Clements with Jane's Defense Group - a British research firm - says it is not the most up-to-date Russian system, but it is quite sophisticated.
"Although there's been a lot of talk about the degradation of the Russian military capabilities since the end of the Soviet Union, in terms of air defense, this is one of the areas where they have held up in terms of the quality and expertise available," Clements says. "So it's a fairly advanced system."
Clements says in 2007, Russia signed a deal with Iran to provide Tehran with the S-300 missile.
"The actual, exact details of the deal itself have never really been released," he notes. "So it's not very clear exactly how many would be sold to Iran, although what we'd be looking at would be Iran trying to purchase enough to be able to defend its main strategic sites, such as its nuclear power station and other primary military sites."
But as of now, as John Parker with the National Defense University says [expressing his personal views], none have been sold to Iran.
"The Russians seemed to start to pull back on prospects for transferring really early last year - 2009 - just about the time that the Obama administration was talking about a 'reset' in relations with Russia," Parker said. "And since then the Russians have not gone forward with the transfers. In the latest United Nations Security Council resolution of June 9, it appears that technically the Russians would not be prevented from transferring the S-300s to Iran."
That resolution, with Russia's consent, imposed new sanctions on Iran over its attempts to build nuclear weapons. Parker says those sanctions include prohibiting the sale to Tehran of offensive weapons such as combat aircraft, battle tanks, attack helicopters and large caliber artillery systems.
"In terms of missiles, it says as defined for the purpose by the United Nation's Register for Conventional Arms," Parker said. "And there, ground-to-air defense missiles, air defensive missiles are not covered."
After some conflicting statements, senior Russian officials - including Prime Minister Vladimir Putin, during a meeting with French President Nicolas Sarkozy - said Moscow has decided to freeze delivery of the S-300 missiles to Tehran.
David Kramer, former senior State Department official in the George W. Bush administration, says Moscow is indicating its displeasure with Tehran's nuclear weapons program.
"I also hope it reflects a recognition in Moscow that should they transfer these missiles, it could lead to war," Kramer said. "Because I don't think Israel is going to stand by and let these missiles become operational in Iran without taking any military action. So I hope Russia understands that and realizes the implications of such a transfer."
Analysts say Iran must feel betrayed by Russia's decision -- first to vote in favor of stronger sanctions and second not to go through with the sale of S-300 missiles. But experts also say Tehran should not seem surprised by Russia's actions because over the past several years, relations between the two countries have considerably soured.