The unexpected statement from Russia's atomic energy ministry appeared to be a direct concession to frequently-expressed US and Israeli concerns that the project could help Iran develop a nuclear weapons program.
Russia and Iran had planned to sign an agreement under which Moscow would provide fuel for the Bushehr nuclear power plant that it has been helping build in southern Iran.
In return, Tehran was to agree to return all of the reactor's spent fuel to Russia for reprocessing, officials in Moscow had said.
Western states, most notably the United States, fear that if Iran keeps the fuel it may later reprocess it to create low-grade nuclear weapons.
The Unites States had pressed Russia not to sign the agreement until Iran allows open inspections by teams from the United Nations of its military installations.
Russia's atomic energy spokesman Alexander Agapov told the Interfax news agency that the new protocol with Iran may not be signed for several more months.
"Generally, all disputes will be resolved by the end of the year," Agapov was quoted as saying.
He blamed the delay on the Iranians.
"The delivery of fuel is constantly being delayed because Iran has no final document on a reaction to a possible emergency" during the transport of fuel, the Russian spokesman said.
"We cannot carry (spent nuclear fuel into Russia) until we are convinced it will be transported safely to a temporary storage facility," he added.
He added that a group of Russian experts would travel to Iran on September 21 to help resolve the problem.
Russia's decision came before a key meeting of the International Atomic Energy Agency (IAEA), the UN's nuclear watchdog, which is due to discuss the Iranian question during September 8-11.