Russia has said a nuclear power station it has been building at Bushehr in southern Iran will not be completed by the end of this year as planned.
Energy Minister Sergei Shmatko said the delay in launching the plant, which will have two pressurised water reactors, was for "technical reasons".
Correspondents say Russia's decision to delay the opening is clearly political.
On Sunday, Russia and the US both warned Iran that time was running out for talks over its nuclear programme.
Speaking after talks in Singapore, US President Barack Obama said it was unfortunate that Iran still seemed unable to say yes to a "creative" international plan to allay suspicions that it is secretly developing nuclear weapons.
His Russian counterpart, Dmitry Medvedev, said he still hoped to persuade Tehran to send its low-enriched uranium to Russia, where it would be further processed to fuel an Iranian research reactor, but warned that "other means" could be employed if progress was not made on the issue.
Iran has failed to give a clear response.
The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), is expected to unveil its latest report on Iran on Monday.
The report will cover findings by inspectors who recently visited the newly-revealed uranium enrichment facility near Qom. Teheran says its nuclear programme is for entirely peaceful purposes.
Russian officials had said earlier this year that the Bushehr plant would be completed before 2010, but on Monday Mr Shmatko said that although progress had been made, there would be no launch. "We expect serious results by the end of the year, but the launch itself will not take place," he told reporters. "The engineers have to reach their findings."
"The building of the Bushehr station is defined absolutely 100% by technological conditions."
The Bushehr nuclear plant was first planned with German help in 1974, but was shelved after the Islamic Revolution. Russia took up the project in 1995, and after numerous delays a timetable for construction was finalised two years ago.
Any nuclear fuel from the plant will be brought from and returned to Russia so that it cannot be used for a weapons programme.
The BBC's Richard Galpin in Moscow says the decision to delay the completion of Bushehr is clearly political - an expression of Russia's frustration at Iran's failure to accept the offer now on the table from the international community.
Russia has other levers over Tehran - it is also delaying the delivery of what could prove to be a crucial air-defence system for Iran, our correspondent says.
The contract for the S-300 surface-to-air missiles was signed two years ago, but Moscow says nothing has been delivered so far.
In recent weeks, top Iranian officials have called for the highly-sophisticated missiles to be handed over.
Speaking on Sunday after talks with Mr Obama, President Medvedev said he was unhappy with the pace of progress on the enrichment deal offered to Iran.
Mr Obama said Iran had failed "so far at least" to respond positively to a deal to send enriched uranium abroad for reprocessing. Russia and France have offered to do this.
Under the plan brokered by the UN nuclear watchdog, the IAEA, and agreed by Russia, the US and France, Iran would send about 1,200kg (2,600lb), or 70%, of its low-enriched uranium, to Russia by the year's end for processing.
Subsequently, France would convert the uranium into fuel rods for use in a reactor in Tehran that produces medical isotopes.
This is seen as a way for Iran to get the fuel it needs, while giving guarantees to the West that it will not be used for nuclear weapons.
Iran has raised "technical and economic considerations" with the IAEA and has missed deadlines to respond.
Iran revealed the existence of the Fordo enrichment facility, which is being built about 30km (20 miles) north of Qom, in September.
Mr Obama's administration has set an end-of-year deadline for serious progress towards a comprehensive solution.
Correspondents say Russia and China are reluctant to agree to new Security Council sanctions, so a coalition of countries, including the EU, might take action themselves.
Iran is already subject to UN sanctions, including financial scrutiny and restrictions on arms imports, and for keeping uranium enrichment activities at its Natanz plant secret.