A senior US official has called on Russia to stop helping Iran build its first civilian nuclear power station.
Russia says it has imposed strict safeguards at Bushehr
After talks in Moscow to discuss Iran's nuclear programme, US envoy Nicholas Burns said other nations should not help Iran, even on civilian projects.
The tension between the US and Russia comes despite attempts to present a united front to Iran.
Tehran has defied UN calls to stop nuclear activity, saying last week it had successfully enriched uranium.
Enriched uranium can be used as fuel in a nuclear plant, or, when highly refined, in a nuclear weapon.
Iran insists its nuclear programme is for peaceful power generation only, while the US and other Western countries believe it is running a covert weapons programme.
Russia's engagement with Iran's nuclear programme has been a source of friction between Moscow and Washington for many years, says the BBC's Russian affairs analyst Steven Eke.
US officials believe it has helped bolster Iranian nuclear know-how.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is chemically processed and converted into a gas by heating it to above 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
"We... think it is important for countries to stop cooperation with Iran on nuclear issues, even on civilian nuclear issues like the Bushehr facility," said Mr Burns, the US undersecretary of state, referring to Iran's first atomic power station in the south of the country.
Russia's impatience with Iran is increasingly visible, says our correspondent, but even so Moscow will not welcome this American intervention.
Russia says it has always maintained adequate safeguards and international oversight would be in place to prevent the diversion of sensitive technologies.
It is also reluctant to do anything that might damage its relationship with Iran, our correspondent says.
The US wants the UN to authorise more robust action against Iran over its nuclear programme, including sanctions.
Moscow has limited itself so far to joining Western calls for Iran to suspend all uranium enrichment.
During the latest talks between major powers in Moscow, moreover, Foreign Minister Sergei Lavrov appeared to specifically rule out discussion of sanctions until the publication of a key report from the head of the UN's nuclear watchdog at the end of April.
In a rare public intervention, General Yuri Baluyevsky, head of the Russian armed forces, suggested there was mounting dissatisfaction with the increasing US pressure for action against Iran.
He stated that Russia would remain strictly neutral in the event of US military action against Iran, but he pledged that Moscow would go ahead with the planned sale of an advanced air-defence system to Tehran.
That provoked a riposte from Mr Burns, who said: "No country should sell weapons to a regime like that."