Russia is helping Iran build a large-scale nuclear reactor, while giving it crucial support at the United Nations. But as Senior Correspondent André de Nesnera reports from Washington, relations between Moscow and Tehran have cooled.

Russia is one of Iran's largest trading partners, providing Tehran with weapons as well as consumer goods. Moscow is also building a nuclear power plant at Bushehr, in southwestern Iran. On the political front, Moscow has been a strong supporter of Iran, opposing any tough sanctions against Tehran over its alleged nuclear-weapons program.

But experts such as Gordon Hahn, who has taught and written extensively about Russia for decades, say despite close economic and political ties, relations between Russia and Iran have been worsening.

"Russia has finally woken up to the fact that Iran was trying to ensnare Moscow in a very close relationship," said Hahn. "I think the main tenet of [Russian President Vladimir] Putin's foreign policy is that Russia should have an independent foreign policy and direct its own course."

Hahn and others say an example of eroding relations between Moscow and Tehran is Russia's decision to delay completion of the Bushehr nuclear power plant.

Russian officials say the delay is due to Iran's problems in paying for Bushehr's construction. But Tehran says its payments have been on time and it accuses Russia of caving in to western pressure to halt its completion. Bushehr was expected to be built by next month - now experts say it could be delayed for more than a year.

Analysts say Russia has also shifted its position regarding international sanctions against Iran for its alleged nuclear-weapons program. The United States and the European Union believe Iran's uranium-enrichment program is designed to ultimately build nuclear arms - Tehran says it only wants to use its enriched uranium for peaceful purposes, such as electricity.

Former Russian foreign ministry official Nikolai Sokov, now at the Monterey Institute of International Studies, says Moscow offered a compromise whereby Russia would enrich uranium on its soil and then send it to Tehran.

"That seemed for the Russians as a nice compromise that would still sort of keep the nuclear program in Iran going and at the same time would remove the objections that the United States and the European Union had," said Sokov. "But the Iranians rejected that compromise and openly said that well, we cannot really trust the Russians that they would still send us fuel for the [Bushehr] power station. And the Russians went back, completely mad at them."

Gordon Hahn and others say initially, Russia was against any kind of sanctions against Iran.

"[It] gradually has shifted to a position where it has supported sanctions, but at the same time, it has tried to defend Iranian interests by watering down those sanctions," said Hahn. "Every time the United Nations Security Council wants to institute sanctions, they have to go begging to Moscow essentially to get them to go along."

Many analysts say it will be interesting to see what role Russia will play in the Security Council as the U.N. body considers stronger sanctions against Iran: whether Moscow is now ready to accept more punitive measures against Tehran as relations between the two nations continue to cool.