Russia warned in March that it would not provide uranium fuel rods for the $1 billion, 1,000 megawatt light water reactor it is building in the Persian Gulf port town of Bushehr as long as Iran continued to ignore UN Security Council demands that it freeze uranium enrichment, diplomats said.

More recently, Moscow has modified that demand, saying no fuel will be provided unless Iran meets another key international request to fully explain past activities that have heightened suspicions it might be developing a nuclear arms programme, two diplomats familiar with Iran’s nuclear file have said.

The two, commenting separately, spoke on condition of anonymity because their information was confidential.

The International Atomic Energy Agency (IAEA), as well as Iranian and Russian Foreign Ministry and nuclear officials, declined comment on Wednesday.

"Russia has told Iran they must cooperate with the agency, to clear the deck on the outstanding issues," one of the Vienna-based diplomats, who earlier worked in Moscow, told Reuters.

"Russia is using the issue of fuel supply to Bushehr to put pressure on the Iranians. Do you think the Russians want to be seen as responsible for letting the Iranians produce a nuclear bomb? Russia will use every trick in their bag to prevent that."

The diplomat said Moscow delivered its newest warning to Tehran around two weeks ago.

Alleged late payments lay groundwork for pressure
Moscow has played a complicated role in attempts to pressure Tehran to comply with international demands, and the newly building pressure represents a stark turn-about in the Kremlin’s relationship to Iran’s nuclear aspirations, which had for many years been sugar coated reiterations by Russian nuclear officialdom that the Islamic Republic was only seeking peaceful nuclear energy.

The first cracks in the cooperative relationship between Moscow and Tehran began to appear earlier this summer as Atomstroiproekt, Russia’s international nuclear reactor building arm, slowed work on the Bushehr reactor, which is now 95 percent complete, over what it said were late payments by Iran on the $1 billion contract.

But one US official, speaking on conditions of anonymity, questioned Russia’s assertion that the construction delays were purely for the advertised financial reasons.

"I’ve seen some stuff that indicates that the delays in providing fuel are more than routine problems over the contract," he said.

Tehran, furthermore, has repeatedly insisted in recent weeks that it has not missed any of its payments to the Russians.

Moscow’s added conditions to Iran that it open the books on its past nuclear activities in exchange for fuel for Bushehr, which had been slated to go online this year, would seem to confirm that Russia shares the western security concerns over a nuclear Iran that it once eschewed.

The reactor’s opening date is now scheduled for fall 2008.

Fingerprints of A.Q Khan?
Other officials from the American and Russian nuclear establishments have indicated in interviews with Bellona Web that much of the nuclear technology in Iran, beyond the Russian-built 1,000 megawatt light water reactor, bears the markings of Abdul Qadeer Khan, the rogue Pakistani physicist who is the mastermind behind that country’s nuclear weapons programme, currently under house arrest for allegedly selling nuclear weapons know-how.

Other nuclear sites in Iran under investigation by the IAEA include a heavy water reactor for the production of plutonium located in Arak. This week, a senior IAEA delegation is in Tehran to nudge the Iranians toward fulfilling a pledge to ease restrictions on access to the Natanz uranium enrichment plant.

Russia still following its own path with Iran
Russia’s brinksmanship with Tehran has more often than not gone against the grain of its western counterparts on the UN Security Council – but nonetheless seems to be achieving the desired results.

For instance, Russia, with China, has blunted attempts by the United States, Britain and France — the three other permanent Security Council members — to impose harsh UN sanctions, leaving Washington no choice but to accept two sets of watered-down penalties.

But Moscow has used the Bushehr reactor as a key leverage point to push compliance with international demands – independently of other Security Council members – such as more openness about its broader nuclear agenda, according to diplomats.

The Security Council still formally demands an end to enrichment, which can be used both to generate power and to make the fissile core of nuclear warheads.

But one of the diplomats, who has deep knowledge of Iran’s file with the UN, told the AP that there was now "tacit understanding" in New York that if Tehran fully cooperates and leaves no IAEA question unanswered, there would be no new UN sanctions – unless the agency probe finds evidence of attempts to make nuclear weapons.

This represents another concession that Washington was forced into by Russia and China.

Iran seen to be in compliance with demands
Even before the increased Russian pressure, Iran agreed to make new concessions in an apparent attempt to stave off new sanctions.

Tehran told the UN nuclear watchdog agency last month that it would answer questions about past experiments and activities that could be linked to a weapons programme. In recent months Iran also has slowed down its enrichment activities and lifted a ban on IAEA inspections of the Arak plutonium reactor, scheduled to go online in 2009.

Past IAEA reports have expressed concerns that Tehran has secretly developed elements of a more sophisticated enrichment program than the one it has made public; that it might not have accounted for all the plutonium it processed in past experiments and that its military might have been involved in enrichment, a programme that Tehran insists is strictly civilian-run.

Revelations that Tehran posses diagrams showing how to form uranium metal into the shape of warheads have heightened concerns.