Officials in Russia and Iran had previously announced plans to switch on the reactor at the southern port of Bushehr this year, giving Iran its first operating nuclear power plant decades after construction started.
"We expect serious results by the end of the year, but the launch itself will not happen," Russia's state-run RIA Novosti news agency quoted Russian Energy Minister Sergei Shmatko as saying. ITAR-Tass and Interfax had similar reports.
According to the reports, Shmatko blamed the delay on technical issues, but analysts say Moscow has used the project to press Tehran to ease its defiance over its nuclear program.
Iran says the program is purely peaceful, while the U.S. and allies claim Tehran is working to develop nuclear weapons. Russia also says Iran must not acquire nuclear weapons, but it has close ties with Tehran and has used its position as a veto-wielding permanent U.N. Security Council member to water down Western-backed sanctions.
Shmatko's remarks came a day after President Barack Obama pushed for continued pressure on Iran and its nuclear program. During talks with his Russian counterpart Dmitry Medvedev in Singapore, Mr. Obama said that "time is running out" for Iran to sign on to a deal with the International Atomic Energy Agency.
Since September, Medvedev has suggested Russia could support further sanctions against if it did not open its nuclear program to inspections to prove it was not trying to build a bomb. He spoke in similar terms Sunday, avoiding the word sanctions but saying "other options remain on the table" if Iran does not meet its obligations.
Meanwhile, the International Atomic Energy Agency reported Monday that Iranian technicians have moved highly sophisticated technical equipment into a previously secret uranium enrichment site in preparation for .
The report offered no estimate of the new plant's capabilities but a senior international official familiar with the watchdog agency's work in Iran said that it appeared designed to produce about a ton of enriched uranium a year.
At Bushehr, Shmatko said construction is proceeding as planned and that Russia "is certain that it will fulfill its commitments to Iran," according to RIA Novosti.
But his remarks raised hackles in Iran, already angry over Russia's foot-dragging on fulfilling a 2007 contract to provide S-300 surface-to-air missiles to Tehran - another deal seen as a Russian lever in relations with Iran.
The semiofficial Mehr news agency quoted the head of Iran's parliamentary committee on national interests and foreign policy, Alaeddin Boroujerdi, as saying that "this hasty expression by (the) Russian energy minister does not look normal."
He urged Iranian nuclear officials to react to the remarks.
Construction of the Bushehr plant started in the 1970s but was abandoned after Iran's 1979 Islamic Revolution. Russia pledged to build it in a $1 billion deal sealed in the mid-1990s.