Russia says it has delayed plans to start up a nuclear reactor in Iran by a year but has stressed it is for technical reasons, not because of external political pressure.
The construction of the controversial $800m Bushehr plant will now start in 2005, according to the Russian Atomic Energy Ministry.
Russia has come under strong pressure from the United States and Israel to drop the project, as Washington accuses Tehran of secretly developing an illegal weapons programme.
But Nikolai Shingaryev, a senior spokesman for the atomic energy ministry, told French news agency AFP that Russian specialists were continuing to draw up a detailed plan for the plant and the start-up was set for 2005.
"The reasons are purely technical, not political," he said.
"There is a huge amount of equipment that is needed. Equipment (that we thought) would work is not going to work."
Russian Foreign Ministry spokesman Alexander Yakovenko told the Russian Itar-Tass news agency that Moscow would "continue co-operation with Iran because it does not run counter the principles of the International Atomic Energy Agency (IAEA)."
Last month the UN nuclear watchdog called on Iran to "suspend all further uranium enrichment-related activities" and to "co-operate fully with the agency to ensure verification of compliance with Iran's safeguards agreement."
News of the delay to the Bushehr plant came after Iran indicated on Monday that it would increase its co-operation with the IAEA in the run-up to the deadline set for Tehran to clarify questions about its nuclear programme.
Iran has continually refused to suspend its nuclear activities, which it says are aimed at developing nuclear energy and not a weapons programme, as suggested by the United States.
Russia has offered to provide Iran with uranium fuel for the plant for 10 years, if it accepts safeguards and returns the spent fuel.
But talks between the two countries have stalled over the details of supplying and returning the spent fuel stalled.
Last month US President George W Bush and Russian President Vladimir Putin, after a Camp David summit, said they shared a common goal of making sure "that Iran doesn't have any nuclear weapon or a nuclear weapons programme".