But Western powers see the deal, reflecting one Iran backed out of six months ago, as overtaken by events including escalating enrichment by Tehran. Along with Russia and China, they have drafted broader sanctions against the Islamic state.
Tehran has threatened to ditch the plan if it is hit with a fourth UN sanctions resolution, envisaged for adoption next month.
Some diplomats say prospects for the deal look bleak unless Iran stops enriching uranium to higher levels, a process it started in February stoking Western fears it ultimately aims to produce bomb-grade material. Iran says its higher enrichment is to produce fuel for the reactor at the center of the swap deal.
The proposal to swap low-enriched uranium (LEU) for fuel to run an Iranian medical research reactor, aimed at allaying fears Iran is trying to amass fissile material for nuclear weapons, was agreed last week by Tehran with Turkey and Brazil.
A letter signed by Iranian nuclear program chief Ali Akbar Salehi was handed over to International Atomic Energy Agency (IAEA) chief Yukiya Amano at a 45-minute meeting in Vienna.
"The director general is conveying the letter to the United States, France and Russia for their consideration," the IAEA said in a statement.
Russia, France and the United States are parties to the original fuel swap pact brokered by Amano's predecessor Mohamed ElBaradei last October but which stumbled on subsequent Iranian demands for sweeping amendments.
As in October, Iran has agreed to transfer 1,200 kg (2,646 lb) of LEU - enough for an atom bomb if enriched to high purity - abroad in return for special fuel rods to replenish the stocks of its medical isotope reactor.
Iran's Arabic-language Al-Alam state television quoted Salehi's letter as saying the deal was "a major step forward" toward defusing tensions over its nuclear energy programme.
Iran's semi-official Fars news agency said the letter "will be the beginning of more detailed negotiations on the fuel swap through the signing of a written agreement.”
Under the deal Iran's LEU would be kept in a third country - Turkey - under IAEA supervision, still officially Iranian property, until Tehran received the fuel rods. Previously the LEU was to be sent to Russia and France for processing, with the fuel rods provided to Iran around a year later.
World powers voiced doubt about the value of the new offer since 1,200 kg no longer represents the bulk of a stockpile that has grown significantly since then. That means Iran could still be left with enough for a nuclear warhead.
Iran has also started refining uranium to a higher level.
"They say the higher enrichment is to make fuel for the reactor. Why would they continue to do this if they get the fuel?" a Western diplomat said. "If they carry on like this then I would be very surprised if this fuel deal goes anywhere."
Iran's envoy to the IAEA, in remarks unlikely to defuse Western suspicions, made clear the fuel swap would not restrict escalating uranium enrichment in Iran in any way. "This is not the issue," Ali Asghar Soltanieh told Reuters.
"We expect (this offer) to be realized as soon as possible," he said in a message to France, Russia and the United States.
Turkey and Brazil - both currently non-permanent members of the Security Council - and Iran have urged a halt to talk of further sanctions because of the deal. But Western powers see it as an Iranian stalling tactic to derail more biting sanctions.
The new, extended sanctions would target Iranian banks and call for inspection of vessels suspected of carrying cargo related to Iran's nuclear or missile programs.
Iran has been under sanctions since 2006 over its refusal to suspend enrichment - technology that can produce fuel for electricity or for atomic bombs - and open up entirely to IAEA inspections and investigations.
