After months of seesawing, Russia's Security Council says it "sees no reason" not to deliver a state-of-the-art air-defense missile system to Iran despite strenuous U.S. and Israeli objections.

But as the Americans and their allies move toward imposing harsh new sanctions on the Islamic Republic because of its nuclear program, it's likely that this is just another diplomatic ploy by the Kremlin to win concessions from Washington.

Russia's support for sanctions is vital but the Kremlin has been equivocating on that in recent days, possibly top exploit the current geopolitical tensions.

Delivery of the powerful missile system would dramatically bolster Iran's defenses against possible U.S. or Israeli airstrikes.

Vladimir Nazarov, deputy secretary of the Security Council, acknowledged Sunday that there was a "signed contract" with Iran but he stressed that delivery of the five batteries of the S-300PMU missiles Iran signed up for had not commenced.

He said that the deal "is not restricted by any international sanctions because we're talking about deliveries of an exclusively defensive weapon."

That seems to suggest that starting deliveries was under consideration by Moscow.

But the Russians have shifted positions on that repeatedly in recent months, first saying it was ready to supply the weapons, then backing off.

Iran has repeatedly demanded that Moscow adhere to the 2007 contract valued at $800 million. It wants the S-300s to protect its key nuclear facilities from threatened airstrikes by Israel.

The Russian statement came as Israeli Prime Minister Binyamin Netanyahu arrived in Moscow for a three-day visit to push the Kremlin to support "crippling sanctions" on Iran. He is also expected to press Russian President Dmitry Medvedev to block delivery of the S-300s.

At present, the Iranian air-defense system does not have anything remotely like the S-300, which can engage multiple targets, missiles as well as aircraft, at a range of more than 100 miles at low and high altitudes.

Israeli aircraft could penetrate Iran's current defense without too much trouble, although there would be losses.

But with S-300s in place it would be immensely more difficult to knock out any nuclear sites and losses would be much higher.

The Americans say that military action is also an option for them but there seems little enthusiasm for that in the administration of President Barack Obama. He has pursued negotiations to persuade Tehran to abandon its nuclear enrichment program, the key step toward acquiring military-grade material for a weapons program, much to Israel's annoyance.

But those talks appear to foundering, heightening tension in the Middle East.

Iran's media reported in December 2008 that deliveries of components of the S-300 system had begun but Moscow swiftly denied that.

On Nov. 12, 2009, Iranian Defense Minister Ahmad Vahidi demanded Russia fulfill the S-300 contract.

On Feb. 5, Iran's ambassador in Moscow, Seyyed Mahmoud-Reza Sajjadi, declared: "Iran is ready to receive these systems and our Russian colleagues have assured us that they will meet their obligations."

But, he added, "several technical issues have emerged" in implementing the contract. "We hope they will be resolved soon." He did not elaborate.

Last Wednesday, the Iranians, clearly furious at the delays, warned through the foreign ministry that failure to deliver the S-300s would "leave a negative imprint in the memory of the Iranian public."

Iran's official news agency quoted Gen. Heshmatollah Kasiri, a senior air force commander, as saying that the Islamic Republic was now "building all our air-defense weapons by ourselves …

"We wanted to import the Russian S-300, which so far they have not delivered for some unacceptable reasons. But our air-defense experts and scientists found a way and in the very near future we will produce an air-defense system which has the capabilities of the S-300 or even more."

The Russians dismissed that out of hand. Valdimir Yevseyev of the Russian Academy of Sciences' World Economics and International Relations Institute, said: "Iran is unable to build these complexes due to its serious technological lag.

"Nor is it likely to acquire such capability in the next five years."

Despite Russia's latest statement, Moscow still appears to be vacillating between its contractual obligations, international pressure and concerns at the potential losses it would incur by not delivering the missiles. These could total around $1 billion in lost profits, plus $300 million-$400 million in fines and penalties.