Iran has rejected claims by Russian officials that it has failed to meet payments for work on the Bushehr nuclear plant in southern Iran.
Electricity was due to come on line at Bushehr in November
Russian officials had warned the $1bn (£513m) deal might be delayed.
Moscow last year backed limited UN sanctions on Iran for refusing to halt uranium enrichment after objections to the Bushehr deal were dropped.
BBC correspondents say Moscow may be wary of delivering nuclear fuel next month, as scheduled in the deal.
Iran insists its nuclear programme is for purely peaceful purposes but the US suspects it of seeking nuclear arms.
Under the Bushehr deal, Russia would have started the fuel shipments by March, launched the plant in September and begun to generate electricity by November.
Russia's Federal Nuclear Power Agency spokesman Sergey Novikov said the "launch schedule definitely could be affected" by the reported delay in payments.
One unnamed Russian official told Associated Press Iran was blaming "technical reasons" for the delay.
But Mohammad Saeedi, the deputy head of the Atomic Energy Organisation of Iran, said: "Iran has had no delay whatsoever in making payments for the Bushehr nuclear power plant to the Russian company."
A UN Security Council deadline is due to expire on Wednesday for Iran to stop the enrichment of uranium.
The United States is pushing hard for the international community to take tough action.
BBC diplomatic correspondent Jonathan Marcus says it would be uncomfortable for the Russians if they began to supply nuclear fuel to Tehran before the International Atomic Energy Agency had given the country's nuclear programme a clean bill of health.
He says many analysts see the reports that Tehran is falling behind on its payments as a pretext to delay the delivery.
The fuel amounts to approximately 100 tonnes of partially enriched uranium.
The BBC's Richard Galpin in Moscow says experts believe Iran has a shortage of uranium and this fuel could be diverted and enriched to weapons-grade material.