An employee works at the fuel manufacturing plant at the Isfahan Uranium Conversion Facility, 440 kilometres south of Tehran, in April. Iran is in talks to turn over most of the low-enriched uranium it has created in the facility. (Caren Firouz/Reuters)

Talks aimed at persuading Iran to hand over the further enrichment of its uranium to an outside country are progressing well, a United Nations official said Monday in Vienna.

Mohamed ElBaradei, who heads the UN's Atomic Energy Agency, said the first day of discussions over the Iran's controversial nuclear program have been productive, and they will continue Tuesday morning.

"We have had this afternoon quite a productive meeting," he told reporters at the end of Monday's closed-door meeting. "We are off to a good start."

Representatives from Iran, the U.S., Russia and France are meeting in Vienna to discuss a plan first agreed to during earlier talks in Geneva. The delegates said little as they left Monday's meetings.

Under the terms of the deal agreed to on Oct. 1, a foreign country, likely Russia, would help enrich uranium for Iran and then send it to France for conversion into metal fuel rods to fuel a small research reactor in Tehran.

Iran has claimed its nuclear program is for energy purposes but the international community has expressed concern it might be used to develop nuclear weapons.

The Islamic Republic has faced a number of sanctions for continuing its program and failing to allow UN inspectors to investigate its nuclear facilities. Western powers had discussed a fourth set of sanctions after it was revealed in September that Iran had begun work on a second uranium enrichment plant.

But during the Geneva discussions, Iran also agreed to allow inspectors to visit the second plant.

Iran could hand over 75% of stockpile

The issue for Monday's talks is how much of Iran's estimated stockpile of low-enriched uranium it is willing to turn over as part of the agreement. The Geneva talks put the tentative quantity at 1,200 kilograms, or as much as 75 per cent of Iran's declared stockpile.

The agreement would be significant because 1,000 kilograms is the commonly accepted threshold of the amount of low-enriched uranium needed for production of weapons-grade uranium enriched to levels above 90 per cent.

Based on the Iranian stockpile, the U.S. has estimated Tehran could produce a nuclear weapon between 2010 and 2015.

Giving up much of its stockpile wouldn't prevent Iran from producing more low-grade uranium in the future, but would slow any attempt to build a weapon, said David Albright of the Washington-based International Institute for Strategic Studies.

"It buys some time," said Albright, who said Iran might be able to replenish its stock in a "little over a year" at its present rate of enrichment.

Iran's state-run Press TV cited unnamed officials in Tehran as saying the Islamic Republic would seek to hold onto its stockpile of uranium while buying what it needed for the reactors from abroad. Such a stance would be considered a major setback to ongoing discussions.

With files from the Associated Press