Russian news agencies reported the reactor at a nuclear power plant that Russia is building in Iran will not be switched on this year as planned, it was announced Monday.
The agencies quote Energy Minister Sergei Shmatko as saying that while Russia expects to make "serious progress" by the end of the year, "there will be no startup."
Officials in Russia and Iran previously announced plans to switch on the reactor this year.
The plant near the city of Bushehr is part of a nuclear program that Iran says is peaceful. The US and Israel claim Iran is working to develop nuclear weapons.
Russia also says Iran must not acquire nuclear weapons, but it has close ties with Teheran and has pledged to complete the more than decade-old project.
Meanwhile, Iranian president Mahmoud Ahmadinejad said that an impasse in nuclear talks between world powers and Iran would only hurt the West by making Teheran push harder to advance its technology.
Ahmadinejad's comments can be seen as a veiled threat that Iran would go ahead with enriching uranium to a higher level should negotiations with the international community fail.
"Cooperation with Iran is in the West's interest" while pressures on the Persian nation would only make the country "more powerful and advanced," Ahmadinejad said, according to a statement posted late Sunday on the presidential Web site.
The remarks come after US President Barack Obama said Iran is running out of time to agree a UN-brokered plan to ship its low-enriched enriched uranium out of the country for further processing.
Ahmadinejad also reiterated that Iran's nuclear rights are not negotiable and that the country's nuclear activities would only continue within the framework of the UN nuclear watchdog. The Web statement did not elaborate on how Western pressure would embolden Iran.
But it's a likely reference to enriching uranium to a higher level of 20 percent, needed to power a research reactor in Teheran that is part of the negotiations with the International Atomic Energy Agency in Vienna.
Iran is currently enriching uranium to less than five percent, which is sufficient to produce fuel for its future nuclear power plant, but has also raised fears in the West of a covert further enrichment by Teheran in a secret nuclear arms pursuit.
Iran says its nuclear program aimed at only peaceful purposes like energy production.