"The message was delivered by the Russians three weeks ago in Tehran," a diplomat, who spoke on condition of confidentiality, told AFP.

Diplomats said this was a new development as it is the first time Russia has put political conditions on its work in Bushehr, a billion-dollar project which Moscow has defended as not a proliferation risk, despite US concerns that Iran could use fuel for the power plant to develop nuclear weapons.

The first diplomat said the Russians had given, in what was a "high-level message," a "broad reaction" to the Iranians, which included complaints over Iran's being in arrears of payments for the reactor in Bushehr, which Tehran says is a key part of its push to use atomic power to generate electricity.

Russian officials had said on July 25 that Bushehr might not be completed until late 2008, which would put off the completion date by a year.

Delivering fuel is the last stage in activating the power plant.

A second diplomat, from a Middle Eastern country, stressed however that the issue was "more than just money."

"The Russians don't want to be seen as the ones helping the Iranians get a nuclear weapon," the diplomat said.

Construction at Bushehr had stalled earlier this year over charges that Iran was not paying its bills for the project.

Iran is defying three UN Security Council resolutions and two sets of Council sanctions for it to stop enriching uranium, which can help make nuclear weapons, and to cooperate with an investigation of its atomic work by the UN watchdog International Atomic Energy Agency.

The Vienna-based IAEA is still unable to declare, after over four years of inspections, that Iran's nuclear program is peaceful.

The United States charges Iran is using its civilian program as a cover for the secret development of nuclear weapons.

Washington points to questions the IAEA is investigating, such as possible work on advanced centrifuges and Iran's possession of blueprints for making the uranium metal hemispheres that are the core of bombs, as signs of military intentions.

Iran has said it is accelerating efforts to resolve these questions.

It allowed last week a visit to a heavy-water reactor it is building in Arak and which can produce plutonium, like enriched uranium a possible atom bomb material.

In addition, four IAEA officials are now in Iran for talks aimed at agreeing a framework for future inspections of the uranium enrichment plant at Natanz in central Iran, the country's most sensitive nuclear site.

Iran expressed hope on Tuesday that its moves to step up cooperation with the IAEA would lead to the West dropping attempts to impose a third set of UN sanctions.

But the central demand of world powers remains that Iran suspend its uranium enrichment activities.

Iran is not about to back down on this.

Iranian President Mahmoud Ahmadinejad said in Algeria Tuesday that "Iran cannot talk to countries which do not recognise its right to produce nuclear energy for peaceful means."

US President George W. Bush this week blasted Iran as "not a force for good" and vowed to pursue efforts to isolate it for its suspect nuclear work.

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