Iranian officials said more talks were needed after they rejected a proposal from Moscow to delay the opening of the plant in the Gulf port of Bushehr until June next year.

"The Russians proposing bringing the Bushehr power station on line in June 2006. We rejected this proposal," Iranian Atomic Energy Organization vice president Mohammad Saidi told state television.

"Discussions will need to continue in Bushehr tomorrow for us to reach agreement."

Russia's top atomic energy official Alexander Rumyantsev and his Iranian counterpart Gholamreza Aghazadeh had been poised to sign the agreement earlier Saturday, but reporters were told that instead negotiations were still "dragging on".

"We do not know when they will conclude," said a spokesman for the Atomic Energy Organisation, Yaghoub Jabarian. He added that a rescheduled press conference would "maybe" take place on Sunday.

It was the latest and most spectacular hitch to a contract that the United States -- which accuses Iran of using an atomic energy drive as a cover for weapons development -- has been trying to convince Russia not to sign.

In a concession to the United States, Moscow had refused to provide fuel for the Bushehr plant in southern Iran unless spent fuel -- which potentially could be reprocessed and upgraded to weapons use -- was returned.

Iran agreed to this condition after close to two years of talks that had already pushed the plant's opening back to next January.

Saidi said he was still hopeful the deal could be signed during Rumyantsev's visit to Bushehr on Sunday.

The deal would cap an 800 million dollar (606-million-euro) contract to build and bring on line the Bushehr reactor.

Bushehr was raised during a summit between US President George W. Bush and Russian President Vladimir Putin in Bratislava on Thursday, where both publicly agreed agreed that Iran should not develop nuclear weapons.

According to Russian diplomats, the United States has been lobbying against Moscow's involvement in Iran's nuclear programme "on a daily basis" -- and right up until the Bratislava meeting.

But they also point out that the huge contract has "virtually saved Russia's atomic energy industry", and emphasise that there is no way Bushehr -- also under International Atomic Energy Agency (IAEA) scrutiny -- could constitute part of a weapons programme.

Russia is also examining the option of building a second reactor at Bushehr along with plants at other locations.

But the United States argues Iran -- a member of President Bush's "axis of evil" -- has no need for nuclear energy because of its massive oil and gas supplies.

Tehran counters that it needs to free up its fossil fuels for export and meet increased energy demands from a burgeoning population. It also denies allegations that it is seeking a bomb, or even the option to build one.

Iran also intends to produce its own nuclear fuel for future plants -- hoped to produce 7,000 megawatts of electricity by 2020 -- a drive at the centre of the current stand-off with the international community.

As a gesture of good faith, Iran agreed last year to suspend enriching uranium used for nuclear fuel, and is currently engaged in negotiations with Britain, France and Germany on a package of trade, security and technology incentives.

But Iran has also insisted that its suspension of fuel cycle work is only a temporary measure.

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