Iran will respond next week to a UN-brokered deal aimed at easing fears about its nuclear programme, rejected a Friday deadline for the agreement on uranium enrichment, the UN atomic watchdog said.

The International Atomic Energy Agency (IAEA) said Iran had asked for more time, but was looking favourably at the deal.

Earlier, France, Russia and the United States, had all given their formal backing to a proposal whereby Russia would enrich the uranium Iran needs for a research reactor that makes radio-isotopes for medical purposes.

Western diplomats had initially said the international powers would not accept any attempt to drag out the negotiations beyond Friday.

However, the United States said that it was now prepared to wait for Iran's reply.

"I think we can stretch things a few days," said State Department spokesman Ian Kelly in Washington following Iran's announcement. "But we're not going to wait forever."

There ws no immediate response from Russia or France.

The IAEA said in a statement Iran had asked for more time to consider the proposal in depth.

"Iran informed Director General (Mohamed ElBaradei) today that it is considering the proposal in depth and in a favourable light, but it needs time until the middle of next week to provide a response," the statement said.

"The Director General hopes that Iran's response will equally be positive, since approval of this agreement will signal a new era of cooperation."

Tehran's envoy to the IAEA, Ali Asghar Soltanieh, told Iranian state television's website that: "Iran is precisely examining different dimensions of the contents of the proposed agreement about the provisional supply of fuel for the Tehran research reactor.

"After final evaluation, I will give the result to Mr. ElBaradei when I return to Vienna next week."

The world has been anxiously waiting all day for Tehran's response to the plan.

The full details of the deal have not been released. But France has said it calls on Iran to hand over 1,200 kilogrammes of low-enriched uranium (LEU) it has at a plant in Natanz -- in defiance of UN sanctions -- to Russia by the end of the year.

Russia would enrich the material to the 19.75 percent needed to use it in a research reactor that makes radio-isotopes for medical use.

Diplomats say Russia would sub-contract to France the process of turning the enriched uranium into the fuel rods for the reactor.

Russia was the first of the four countries to formally back the proposal. It was followed by France and the United States.

In Tehran, an Iranian source close to the Vienna talks was quoted as saying on the state television's website that Iran had wanted a "positive" response to its own proposal to procure nuclear fuel for the reactor.

"Iran is the buyer of nuclear fuel for the Tehran reactor and sellers must give a positive response to the buyer's proposal and not consider their own view as a positive response," the source said.

Iran "entered the Vienna talks with a positive and constructive approach, and now it is awaiting a constructive response and positive response to Iran's proposal."

The other parties "are expected to give a response which builds confidence and is constructive in return for the transparency and goodwill of the Islamic republic," the source said.

Uranium enrichment lies at the heart of Western concerns about Iran's nuclear programme. It produces fuel for civilian reactors, but in highly extended form can also make the fissile core of an atomic bomb.

On Saturday, IAEA inspectors are set to fly out to Iran to inspect a hitherto undeclared second uranium enrichment plant near the holy city of Qom.

According to the official IRNA news agency, inspectors will spend two or three days in Iran, which denies Western allegations it is seeking to build an atomic bomb.

Iran insists it will not halt its enrichment work at Natanz even if it approves the deal.

It has also indicated that it does not want France involved in the arrangement. France, among the western powers, has taken a tough line opposing Iran's nuclear programme.