The agreement came after seven and a half hours of talks between Iran and world powers just outside Geneva, where Tehran also agreed to grant the United Nations access to a newly-revealed covert uranium enrichment plant near the holy city of Qom.

The meeting at a Swiss villa included the highest-level direct talks between the United States and Iran in three decades.

Under the tentative plan, Iran would ship the bulk of its stockpile of low-enriched uranium (LEU), which is enriched to about 3.5 percent, to Russia, where it would be further refined to 19.75 purity, still far below nuclear weapons grade, a US official told reporters in Geneva.

French technicians would then use the material to produce fuel rods that would be returned to Iran to feed the reactor, which Tehran says will run out of fuel in the 12 to 18 months, added the official, who spoke on condition of anonymity due to the sensitivity of the matter.

“The potential advantage of this, if it’s implemented, is that it would significantly reduce Iran’s LEU stockpile which itself is a source of anxiety in the Middle East and elsewhere,” said the official.

Iran, the official said, came to the International Atomic Energy Agency (IAEA) “a few months ago” with a request to replace the Tehran reactor’s fuel supply, last provided by the Argentine government in the early 1990s.

The IAEA then conveyed to the Iranians a joint US and Russian proposal under which Tehran could use its own LEU stockpile as the basis for its needed reactor fuel.

The official hailed the plan as a “positive interim step to help build confidence” on Iran’s controversial nuclear program following the talks between the Islamic Republic and the five permanent members of the UN Security Council — China, Russia, France, Britain and the United States — plus Germany.

IAEA chief Mohamed ElBaradei is set to travel to Tehran this weekend to iron out the details of the plan and the powers and Iran will reconvene at a meeting in Vienna on October 18 led by experts from the UN nuclear watchdog.