The issue has dogged Russian-US relations for years and is likely to surface again at next month's Camp David summit between presidents George W. Bush and Vladimir Putin -- just as it did during their last meeting in Saint Petersburg on June 1.
John Bolton, the US under-secretary of state for arms control and international security, spent two days in Moscow holding talks with senior Russian officials responsible for non-proliferation and nuclear issues.
Part of the talks focused on Wednesday's key six-way meeting on the North Korean nuclear crisis -- in which Bolton will not take part -- but much attention was also given to Russia's decision to press ahead with construction of Iran's first nuclear power plant at Bushehr.
The US fears that Iran is using the project to secretly develop a nuclear weapons program. Moscow counters that Tehran may be building such a program with Western companies' help.
And Russian reports said both sides stuck firmly to their positions in Moscow.
"There were no new developments on the subject of Iran at Bolton's consultations at the foreign ministry," a Russian diplomatic source told the Interfax news agency, referring to the US official's meeting on Monday with Deputy Foreign Minister Sergei Kislyak.
"The American side confirmed its concern about the Iranian nuclear program and the Russian side once again spoke of the complete transparency of its nuclear cooperation with Tehran," the source said.
But worse news followed for Washington after Bolton's departure when the ITAR-TASS agency reported that Russian officials had submitted documents to Iran requesting permission to launch the construction of a second reactor at Bushehr.
Washington had earlier expressed optimism that Moscow would go ahead and complete the first reactor -- but that its cooperation with Tehran would end there.
US officials have begun to claim some success in that area. State Department spokesman Richard Boucher said only last week that Russia's position seems to have shifted.
Senior US diplomats in Moscow have also said in private that Atomic Energy Minister Alexander Rumyantsev -- who met Bolton on Tuesday -- has recently appeared to soften his position concerning Iran.
But that speculation appeared to have been dashed by the latest meetings in Moscow.
An atomic energy ministry spokesman told AFP that it was "very likely" that Russia and Iran would sign a key protocol agreement next month in which Tehran would promise to return all spent nuclear fuel to Russia for reprocessing.
The West had expressed concern that the spent fuel could be enriched to create low-grade nuclear weapons if it were kept by Iran.
Tehran is under further pressure from the international community to sign an additional protocol to the nuclear Non-Proliferation Treaty that would allow UN staff to carry snap inspections of suspected Iranian nuclear sites.
This was one issue that Bolton and Rumyantsev appeared to agree on.
"Moscow and Washington have a common understanding on the need to work for Iran's greater openness in the nuclear sphere," a Russian diplomat told Interfax.
But the Russian media still speculated the United States had lost all leverage over Moscow's cooperation with Tehran now that the return-of-fuel agreement is about to be signed.
Next month's planned signature of the protocol "takes away the last trump card the US held for Moscow giving up support for Iran's nuclear program," the Kommersant business daily opined.