Iran agreed on Monday to ship much of its low enriched uranium abroad in a nuclear fuel swap deal backed by Turkey and Brazil but the United States said moves for toughened sanctions would still go ahead.

The accord, which commits Iran to depositing 1,200 kilograms (2,640 pounds) of low enriched uranium (LEU) in Turkey in return for fuel for a research reactor, was signed by the foreign ministers of Iran, Turkey and Brazil.

Tehran, already under three sets of UN sanctions over its defiant nuclear drive, touted the agreement as a goodwill gesture that paves the way for a resumption of talks with world powers.

But the United States said it would not halt or slow its drive for toughened sanctions against the Islamic republic.

"It does not change the steps that we are taking to hold Iran responsible for its obligations, including sanctions," White House spokesman Robert Gibbs said.

Gibbs, who had earlier issued a written statement expressing "serious concerns" about the deal, said that if Tehran lived up to the new pact it would represent "some progress."

But even if that took place, Gibbs said Washington had concerns about the "overall thrust" of the nuclear programme and the fact Tehran said it would continue enriching uranium to 20 percent.

Turkish Foreign Minister Ahmed Davutoglu said there was "no need" for further UN sanctions in the light of the deal as his country and Brazil "have made guarantees and the low enriched uranium will remain in Turkey."

Brazilian President Luiz Inacio Lula da Silva, a driving force behind the accord, said: "Diplomacy emerged victorious today. It showed that it is possible to build peace and development with dialogue."

Monday's signing came after three-way talks in Tehran by President Mahmoud Ahmadinejad, Lula and Turkish Prime Minister Recep Tayyip Erdogan.

Lula and Erdogan are due to meet again on Tuesday in Madrid on the sidelines of the EU-Latin America summit, according to the Brazilian president's agenda.

EU foreign affairs chief Catherine Ashton said Monday's accord only "partly" responds to the demands of the International Atomic Energy Agency (IAEA), which has been probing Iran's nuclear programme for years.

The IAEA said it had received the text of the joint declaration by Iran, Brazil and Turkey, but was now expecting Tehran to notify it directly of what commitments it had undertaken.

"In line with what was stated there, we are now expecting written notification from Iran that it agrees with the relevant provisions included in the declaration," said spokeswoman Gill Tudor.

Ashton said an IAEA offer in October for Iran to have its uranium enriched abroad recognised the right for civil nuclear power, but was "not an attempt to address the underlying issue... which is the nuclear weapon intention issue."

Russian President Dmitry Medvedev had previously welcomed the agreement but said further talks were needed.

"What was done by our colleagues needs to be welcomed. This is the politics of a diplomatic solution," he said. "We need to have consultations with all the parties, including Iran, and then determine what to do next."

Iran had said the signing meant the ball was now in the court of Western powers, and Ahmadinejad called for fresh talks over Tehran's nuclear programme.

"I hope the 5+1 (UN Security Council permanent members plus Germany) enter talks with honesty, respect and fairness and heed the great work started in Tehran," the official IRNA news agency quoted him as saying.

Iran's arch-foe Israel -- the sole if undeclared nuclear-armed power in the Middle East -- was quick to accuse Tehran of manipulating Turkey and Brazil and seeking to buy time in the long-running nuclear standoff.

Britain also expressed reservations. "Iran's actions remain a serious cause for concern," junior foreign minister Alistair Burt said.

Germany said nothing could replace the IAEA deal that envisages Iran's 3.5 percent LEU being sent to Russia and France for enrichment to 20 percent and then returned as fuel for the Tehran reactor.

The UN-brokered offer appeared to be a breakthrough in long-stalled discussions over the refuelling of the Tehran research reactor that makes radioisotopes for cancer treatment.

But Iran has so far stalled on the deal the IAEA has been trying to persuade it to sign since October, insisting it wants to keep the uranium on its own soil for a simultaneous swap with reactor fuel.

Enriched uranium is the focus of Western suspicions over Tehran's atomic programme, because in a highly purified form it can be used to make the fissile material of a nuclear bomb.

Under the new agreement, a copy of which was obtained by AFP, Tehran is ready to deposit more than half of its LEU in Turkey "within one month."

In return, the United States, France and Russia would deliver 120 kilos of fuel needed for the reactor "in no later than one year."

An Israeli official told AFP the fuel swap accord would "radically complicate" international efforts to rein in Iran's nuclear programme through sanctions due to the involvement of rising powers such as Brazil and Turkey.

Tehran sparked international concerns in February by stepping up its enrichment level to 20 percent to make fuel for the research reactor. At the time, the UN watchdog calculated that Iran had stockpiled 2,065 kilos of LEU.