Iran has begun large-scale conversion, Iranian atomic energy chief Reza Aghazadeh said in Vienna Tuesday, going against a UN call for it to suspend conversion and all other activities that support uranium enrichment, a process that can make material for nuclear weapons.

The UN's Vienna-based International Atomic Energy Agency (IAEA) had adopted a resolution Saturday for Iran to "immediately" suspend all parts of the nuclear fuel cycle.

The IAEA set a deadline of November 25, when its board of governors will next meet, for a definitive review of Iran's nuclear program, which the United States claims hides covert weapons development.

The United States is pushing for Iran to be sent to the UN Security Council, which can impose punishing sanctions.

Iran is defying the IAEA while engaging in an "unrelenting push toward nuclear weapons capability," the United States said Tuesday.

"It should come as no surprise that Iran has defied the board (of the IAEA) once again and announced it is producing uranium hexafluoride (gas), the material for centrifuge enrichment," Kurtis Cooper, a State Department spokesman, said.

"Although Iran has repeatedly asserted that its nuclear program is exclusively for peaceful purposes and its pursuit of uranium enrichment technologies are to fuel a planned civilian power program, Iran will have no peaceful use for enriched uranium for many, many years," he said.

"The rush to convert 37 tons of yellowcake into feed-stock for centrifuge enrichment has no peaceful justification," the spokesman said. "Iran has no operating nuclear power plants.

Enriched uranium can be fuel for civilian reactors, but also the explosive core for atomic bombs.

Aghazadeh told reporters in Vienna, where the IAEA is holding a general conference, that "some of the 37 tonnes" of uranium yellowcake, or ore, which Iran had previously said it would be converting had now been used.

"Tests have been successful but these tests have to be continued using the rest of this material," Aghazadeh said.

Yellowcake is converted into uranium gas which is then fed into centrifuges to make enriched uranium.

The United States said that industrial-scale uranium conversion, even if Tehran only designated this as "tests", would be an alarming sign that Iran is continuing its quest for a bomb.

"Well, I think that the clock is ticking. I think the Iranians have determined their course of action and are going with it," John Pike of the Washington-based Global Security think tank told AFP.

Aghazadeh said Iran did not recognize the validity of the IAEA demand for Tehran to halt uranium enrichment, as it is allowed under the nuclear Non-Proliferation Treaty (NPT), whose rules the watchdog monitors.

He reiterated Iran's position that its suspension of uranium enrichment, agreed with Britain, France and Germany in October 2003, was "based on a voluntary gesture."

While Iran has suspended actual enrichment, it has not suspended support activities, such as making centrifuges and converting yellowcake into UF6 gas.

The IAEA had already noted on September 1 that Iran had indicated it would resume large-scale production of uranium gas.

In a report, it said Tehran plans "a large-scale test of (converting) 37 metric tonnes of yellowcake (uranium in mineral form)" into uranium hexafluoride, the gaseous "feed-stock" for enriching uranium to what can be bomb-grade levels.

Iran had conducted a smaller test in May and June 2004 for making UF6, the report said.

Experts close to the IAEA said then that upcoming production of the uranium hexafluoride would produce a "significant amount" of the gas, an amount that would be enough to produce enough enriched uranium that could produce at least one atomic bomb.

US officials say that if all 37 tonnes of yellowcake were converted, that could produce enough enriched uranium to make up to five bombs.

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