Iran says it has delayed plans to start up its first nuclear reactor by a year - but it plans to build more plants with Russian help.
The US has urged Russia to halt construction at Bushehr
A top atomic energy official said the reactor in the southern town of Bushehr is now due to enter service in 2006.
Russia had agreed to build at least one more reactor at the site and two other European countries had shown interest in the projects, the official added.
Tehran denies US claims the Bushehr reactor could be used to make weapons.
The US - which says Iran has been running a secret nuclear weapons programme - has put pressure on Russia not to go ahead with the first reactor in Iran.
It also accuses Iran of using a site at Lavizan for nuclear weapons research.
Tehran says its nuclear programme is designed solely to provide energy.
Construction on the Bushehr reactor finally began in 2002, more than two decades after the building plan was first agreed with West Germany before the 1979 Islamic revolution.
The plant, which is being repaired and redesigned by Russians, has had its start date pushed back steadily in recent years.
Asadollah Sabouri, deputy head of Iran's Atomic Energy Organisation, said the plant is now expected to be operational by late 2006.
One reason for the delay is attention to international standards on safety and the
environment, he said.
Correspondents say another factor is the failure
to agree on a contract to return spent fuel to Russia.
The agreement is reportedly designed to ease fears that Iran could reprocess
the spent fuel and turn it into bomb-grade material.
Mr Sabouri said Iran and Russia had yet to agree on the cost
and procedures for returning the spent fuel, but were
committed to reaching a deal, Reuters news agency reported.
"We have contracts with Russia to build more nuclear
reactors, " he added.
Mr Sabouri "welcomed" the European countries' expressions "of readiness to join their projects", but refused to name them.