Q+A: Why did Russia fall out with Iran?

MOSCOW (Reuters) - Iran's tirade against Russia on Wednesday for supporting fresh U.N. sanctions showed how the former allies have now publicly fallen out.

President Mahmoud Ahmadinejad issued a rare public rebuke to his Russian counterpart Dmitry Medvedev, telling him he should "act more cautiously" and "think more." The Kremlin replied that Ahmadinejad should refrain from "political demagoguery."

Below are some questions and answers on why and how Moscow has shifted its position on Iran:

WEREN'T IRAN AND RUSSIA ALLIES ?

Russia is a significant trading partner with Iran. Bilateral trade reached $3 billion last year, with Moscow selling the Islamic Republic nuclear technology, aircraft and other goods.

Iran and Russia are also among the world's top oil and gas producers and have cooperated in this area.

In the diplomatic arena, Russia had resisted in 2008 and early 2009 fresh U.N. sanctions against Tehran and played down suggestions Iran was using its nuclear program to build bombs.

HAS RUSSIA OPPOSED AN IRANIAN BOMB ?

Russian officials have always insisted Moscow -- which has a big problem of its own with Islamist terrorism -- does not want to see a powerful Islamic state near its troubled southern borders acquire nuclear weapons. But until last year, Russia didn't believe American assessments that it was likely to happen.

DIDN'T PUTIN DISMISS ANY NUCLEAR THREAT FROM IRAN ?

In October 2007 while still president, Vladimir Putin became the first Kremlin leader to visit Iran since Stalin, delivering smiling support to Ahmadinejad, warning the United States against any military action and upholding Iran's right to pursue a civilian nuclear program.

DID RUSSIA BELIEVE IRAN WAS PURSUING A BOMB?

U.S. Defense Secretary Robert Gates recounted a meeting he had in 2007 with Putin:

"When I first met with President Putin and talked about this, he basically dismissed the idea that the Iranians would have a missile that would have the range to reach much of Western Europe and much of Russia before 2020 or so," he said in testimony to the U.S. Senate last year.

"And he showed me a map that his intelligence guys had prepared. And I told him he needed a new intelligence service."

WHEN DID THIS START TO CHANGE?

In the two years after that meeting, Russia started to change its assessment of the Iranian program. However, in June 2009 Moscow was still happy to welcome Ahmadinejad to a summit of BRIC nations in Siberia and congratulate him on his disputed re-election. The real shift in policy toward Iran appears to have started over the course of last summer in Moscow.

DID OBAMA'S ELECTION ALTER ANYTHING?

When President Obama came to power in January 2009, he vowed to "reset" relations with Russia. This meant concessions to Moscow such as scaling back Bush-era missile Defense plans in eastern Europe and accepting Russian influence in the former Soviet Union, in return for Moscow's help on tackling international problems such as the Iranian nuclear program and Afghanistan. Ties between the two nations improved dramatically.

BUT WASN'T RUSSIA RESISTING SANCTIONS ONLY LAST YEAR?

Despite headlines from Russian officials apparently resisting Western pressure on Iran, Western ambassadors in Moscow were talking confidently last year about how helpful and supportive Russia had been on Iran. It appears that Moscow was giving private assurances of support to the West on Iran some time before it changed its public position.

The West's announcement in September that it had discovered a new secret Iranian nuclear fuel plant near the Muslim holy city of Qom further undermined Moscow's confidence in Iran. Russia said the plant violated U.N. Security Council decisions and was a "source of serious concern." In November 2009, Moscow supported an IAEA resolution condemning the move.

DID MEDVEDEV MAKE A DIFFERENCE?

Medvedev's strong personal relationship with Obama has made it easier for the two leaders to agree a common position on Iran. The Russian president first started talking of fresh sanctions against Iran last September and mentioned them again during a visit to the United State the same month.

After signing a nuclear arms reduction treaty with Obama last month, Medvedev said he regretted that Iran was not reacting to constructive proposals on its nuclear program. Iran has complained that Russia is caving in to U.S. pressure.

ISN'T RUSSIA STILL PROVIDING NUCLEAR TECHNOLOGY TO IRAN?

Moscow has a $1 billion contract with Tehran to build and start up a nuclear power plant at Bushehr. The plant is planned to start up in August after numerous delays -- which a senior Iranian lawmaker said were the result of Russia using Iran as a pawn in dealings with other powers such as the United States.

U.S. Secretary of State Hillary Clinton complained about Bushehr's planned summer start-up when she visited Moscow in March, but Western diplomats say privately Russia has offered satisfactory safeguards against the plant being used for military purposes.

AND WASN'T RUSSIA GOING TO SELL IRAN AN AIR Defense SYSTEM?

Moscow signed a contract in 2007 to sell Iran the S-300, a modern surface-to-air missile system that can be used to shoot down multiple hostile rockets and aircraft. However Russia has not yet fulfilled the contract and Western envoys say they have private assurances from Moscow that it will not do so.

WHAT MADE RUSSIA AGREE TO SANCTIONS AGAINST IRAN THIS TIME?

A senior Kremlin official said earlier this month that if Washington wanted Moscow's support for fresh sanctions against Iran, it needed to drop U.S. bans on trade with four Russian arms companies. Washington dropped the bans on Friday last week, though U.S. officials continue to deny any direct linkage with the Iran sanctions issue.

(Editing by Ralph Boulton)