Russia on Monday said it would not start a nuclear reactor at Iran's Bushehr atomic power station by the end of the year as planned, citing technical reasons.
"The launch will not happen by the end of the year," Russian Energy Minister Sergei Shmatko told reporters. "The engineers have to reach their findings."
Russia's nuclear chief Sergei Kiriyenko had said in February that the Bushehr launch was scheduled for 2009 but the plant has been delayed frequently.
Shmatko said the Bushehr decision was purely technical and that Russia would fulfill its contractual obligations to Iran.
But diplomats say Russia uses Bushehr as a lever in relations with Tehran, which is suspected by the United States and other Western powers of seeking to build a nuclear weapon.
Russian President Dmitry Medvedev said on Sunday after meeting US leader Barack Obama in Singapore that Moscow was not completely happy about the pace of dialogue between Iran and the international community over Tehran's nuclear program.
Russia started deliveries of nuclear fuel for Bushehr in late 2007, a step both Washington and Moscow said removed any need for Iran to have its own uranium enrichment program. Russia last year completed delivery of fuel under a contract estimated to be worth about $1 billion.
Russia says the plant is purely civilian and cannot be used for any weapons program, as Iran will return all spent fuel rods to Russia.