Tehran has said it needs enriched uranium for nuclear fuel. But the West fears it could be used to make weapons, and the U.S. says Iran is now one to six years away from being able to do so.
Monday's Vienna talks between Iran and the U.S., Russia and France were focused on a technical issue with huge strategic ramifications - whether Iran is ready to farm out some of its uranium enrichment program to a foreign country.
ElBaradei appeared cautiously optimistic after the first day of closed meetings, saying most technical issues had been discussed and the parties would meet again Tuesday morning.
"We have had this afternoon quite a constructive meeting," ElBaradei told reporters. "We are off to a good start."
The delegations said little as they left the meeting. The chief Iranian delegate, Ali Asghar Soltanieh, said only that he endorsed ElBaradei's comments.
Iran's state-run Press TV cited unnamed officials in Tehran as saying the Islamic Republic was looking to keep its low-enriched uranium and buy what it needed for the Tehran reactor abroad. One source said Iran was looking to the U.S., Russia or France for such supplies - a stance that would likely doom the talks, with neither the U.S. or France accepting such terms.
Tehran's refusal to give up most of its enriched stock could also abort chances of a second round of broader negotiations between Tehran and six world powers.
Western officials were attempting Monday to implement what they said Iran had agreed to during Oct. 1 talks in Geneva - letting a foreign country, most likely Russia, turn most of its low-enriched uranium into higher grades to fuel its small research reactor in Tehran.
That would mean turning over more than 1,200 kilograms (2,600 pounds) of low-enriched uranium - as much as 75 percent of Iran's declared stockpile. Tentative plans would be for further enrichment in Russia and then conversion in France into metal fuel rods for the Tehran reactor.
Iran agreeing to ship most of its enriched uranium abroad would be significant in easing Western fears about Iran's nuclear program, as 1,000 kilograms (2,205 pounds) is the commonly accepted threshold of the amount of low-enriched uranium needed for production of weapons-grade uranium enriched to levels above 90 percent.
Based on the present Iranian stockpile, the U.S. has estimated that Tehran could produce a nuclear weapon between 2010 and 2015, an assessment that broadly jibes with those from Israel and other nations tracking Tehran's nuclear program.
If most of Iran's declared stock is taken out of the country, further enriched abroad and then turned into fuel for the Tehran reactor, any effort to make nuclear weapons would be delayed until Iran again has enriched enough material to turn into weapons-grade uranium.
"It buys some time," said David Albright of the Washington-based IISS, which has closely tracked Iran for signs of any covert proliferation. But Albright said Iran could replace even 1,200 kilograms (2,600 pounds) of low-enriched uranium "in little over a year." Iran now has more than 4,000 centrifuges producing low-enriched uranium, and its capacities are increasing.
Tehran, if it agrees to ship out the enriched uranium, could also resist pressure to hand over most of its stock in one batch, and instead seek to send small amounts at a time. Iran has enough fuel for the Tehran reactor to last until mid-2011.
Before the talks, it was unclear if Iran was even open to discussing shipping out most of its enriched stock.
Press TV, in a report before the talks began, quoted an unnamed source as saying Iran would not negotiate with France because Tehran has not received any enriched uranium from France, despite owning 10 percent of that nation's Eurodif nuclear plant. Areva, the state-run French nuclear company, has described Iran as a "sleeping partner" in Eurodif, which Tehran bought into more than three decades ago.
There were no signs, however, that the French delegation was shut out of the closed-door meeting Monday.
The six powers - the U.S., Russia, China, Britain, France and Germany - have tentatively scheduled follow-up talks by the end of this month aimed at starting negotiations that will ultimately place strict controls on Iran's enrichment activities.
In Iran, leaders have accused Britain and the United States - as well as Pakistan - of aiding the Sunni insurgent group Jundallah, or Soldiers of God, which claimed responsibility for a suicide bombing Sunday that killed five Revolutionary Guard commanders and at least 37 others.
Iran routinely blames Western powers for having links to internal upheavals, including the protests and clashes that followed June's disputed re-election of President Mahmoud Ahmadinejad. The allegations are rarely mentioned by Iran's nuclear negotiators, and have not altered Iran positions such as the demand to enrich uranium.
Associated Press Writer Brian Murphy in Dubai, United Arab Emirates, contributed to this report.