In half a decade of nuclear crisis, Iran's President Mahmoud Ahmadinejad has counted on the Russia of strongman Vladimir Putin to balance the hard line of the West with a more moderate stance.

But in the last weeks, senior Russian figures have signalled mounting frustration with Iran, saying that new sanctions could be realistic and even casting doubt on Tehran's insistence that its nuclear drive is peaceful.

With the United States courting Moscow on the subject, speculation has grown that the previously unthinkable might happen -- Russia joining the West in agreeing sanctions that would threaten the Iranian economy.

Analysts caution however that while there has been an unprecedented shift in Russia's rhetoric on Iran, this does not equate to a wholesale change in policy that could see it back tough measures against the Iranian oil industry.

The position of Russia, which has the closest contacts with Iran of any major power, is crucial. It is a veto-bearing UN Security Council permanent member and also has an unmatched capacity to influence Tehran.

"In the last months it is true that a lot has changed in the behaviour of Russia towards Iran," Rajab Safarov, director of the Centre for Contemporary Iranian Studies in Moscow, told AFP.

"But it is the emphasis and the tone that have shifted, while Russia's overall position on Iran has not changed," he added.

"These statements are an attempt to put pressure on Iran to make it more open to negotiations."

Russia's chief worry in the nuclear crisis was preventing any dramatic escalation of regional tensions, given that its southern border lies just 150 kilometres (100 miles) from Iran, said Safarov.

Prime Minister Benjamin Netanyahu of Israel, Iran's arch enemy in the region, was in Moscow last week to seek Russian support for "biting" sanctions against Iran that would hit the oil industry, its foreign currency lifeblood.

But while President Dmitry Medvedev has since September 2009 repeatedly said that sanctions could not be ruled out, Russia appears to be some way from backing Western calls for tough economic punishment.

Safarov said: "In spite of its threatening statements, Russia would not support a Security Council resolution for new sanctions if there was one now."

Russia is in "a slightly different place" to a few months ago, said one Western diplomat, asking not to be named. "But there is still a huge process to go through."

With Russia often finding itself sidelined in post-Cold War diplomacy, the prolongation of the Iran standoff allows it to flex its muscles on a big issue where it unquestionably remains a player.

"Russia has an interest in the issue remaining in suspense," said Alexei Malashenko of the Carnegie Centre in Moscow.

"If Ahmadinejad gives in to the pressure, Russia will first get the credit but then its role would diminish.

"Russia will never vote for economic sanctions at the UN Security Council against Iran as it would lose its specific role."

With Russia's position crucial, one man has kept a careful public silence. The last major policy statement on Iran from Putin -- who in 2007 became the first Kremlin chief to visit Tehran in the Islamic Republic's history -- dates back to October.

But lower-ranking figures have made statements that would have been unimaginable just months before.

Iran is "always changing its conditions" and Russia's fears were now "not so far away" from those of Europe and the United States, said parliament's foreign affairs committee chief Konstantin Kosachev last month.

A string of unpleasant surprises has given Russia good reason to revise its tone on Iran.

Russia -- which prides itself on having intelligence sources inside Iran far superior to those of the West -- was taken aback by Tehran's revelation in September that it had built a new secret nuclear plant.

Along with France and the United States, it is also a key player in a deal brokered by the UN nuclear watchdog, the International Atomic Energy Agency, that aimed to defuse the standoff by enriching Iranian uranium abroad.

Iran so far appears to have rejected the deal, a defiance that Russian Foreign Minister Sergei Lavrov has openly criticised as regrettable.

In the energy-sapping game of nerves Iran is playing against the international community, Russia still holds some powerful pieces that give it a leverage on Tehran that no-one else can boast.

Chief among these are five sophisticated S-300 air defence missile systems which Russia agreed to sell to Iran for a reported 800 million dollars several years ago but has never delivered.

Russia is also building Iran's first nuclear power plant in the southern city of Bushehr, a much-delayed project dating back to the shah's era that is finally due to come online this year.