"The Iranians have sought to accomplish a technological achievement for political purposes and chose the Chinese feedstock gas because of its quality, which ensures a better enrichment process," said a diplomat with access to intelligence sources.

The diplomat, who asked not to be named due to the sensitivity of the issue, said Iran had "wanted to declare it had done uranium enrichment and were in a hurry," as they wanted to have a fait accompli before the UN Security Council could move against them.

A second diplomat said Iran had indeed used UF6 supplied by China but had also tried out some of its own feedstock gas -- which intelligence sources say is believed to contain contaminants that can cause the centrifuges used in enrichment to crash.

The Security Council had called on Iran on March 29 to halt enrichment, which makes fuel for nuclear power reactors but can also produce the raw material for atomic bombs.

The Iranians "did not use their own UF6 because they wanted to be completely sure" they could turn out enriched uranium in time, the first diplomat said, referring to uranium ore converted into uranium hexafluoride gas (UF6), which is the feedstock for making enriched uranium.

Iran defied the Council's calls, and the world body is now deadlocked over whether to issue a resolution that would legally oblige Iran to stop uranium enrichment.

Iran's hardline President Mahmoud Ahmadinejad on Wednesday ridiculed an EU plan to offer trade and technology incentives in exchange for an agreement to halt the highly strategic enrichment work.

Iran had suspended enrichment-related work as part of talks with the European Union since October 2003 on guaranteeing that its nuclear program is peaceful but began making UF6 again last August when talks broke down.

By September, they had made some 110 tons of the gas, according to a report of the UN watchdog International Atomic Energy Agency (IAEA).

If the entire quantity were enriched, it would yield enough material for over 10 atom bombs, experts said.

Iran began feeding UF6 gas into centrifuges in February, thus beginning the enrichment process, at a facility in Natanz in the center of the country.

On April 11, Tehran announced that it had actually made enriched uranium but only to levels appropriate for reactor fuel, not for weapons.

The first diplomat said that Iran had used only a few tons of UF6 and had made only "dozens of grams" of enriched uranium, far from the 15-25 kilograms (30-55 pounds) needed to make a nuclear bomb.

"It is a technological success, but it is politically that it is very important," the diplomat said.

China began building a conversion facility in Isfahan in the 1990s to make UF6 but broke the contract in 1997 under US pressure.

Iran completed the facility using Chinese designs.

The second diplomat said the Iranians used Chinese feed but also their own UF6, made in Isfahan, at the Natanz enrichment facility, where they had completed a 164-centrifuge cascade.

"We think they used both, perhaps to compare the two, and certainly to demonstrate to themselves that their own UF6 is capable of being enriched without too many centrifuge problems," the diplomat said.

The diplomats said Iran's success in moving ahead on enrichment after an almost three-year-long suspension was a sign that they carried out secret work during the halt.

In 2003, the Iranians "did not manage to run even one centrifuge for a period of time" and now "when they started this year they are very, very good," the first diplomat said.

"They are feeding the UF6 in the right quantities, they built a 164-centrifuge cascade in a few days, and they are enriching to 4.8 percent," the diplomat said, adding: "The question is, 'Where did all that know-how come from?'"

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