Defying international concerns, Iran (search) has resumed clandestine work linked to uranium enrichment, testing equipment and producing a gas that can be used to make nuclear warheads, diplomats said Wednesday.

The diplomats told The Associated Press that Tehran has restarted equipment used to make uranium hexaflouride gas (search), which, when injected into centrifuges and spun, can be enriched to a level high enough to make the weapons.

While Iran only appears to be testing the machinery, it has apparently produced some of the gas as a side effect, said the diplomats, who are either familiar with Vienna-based International Atomic Energy Agency's investigations or privy to intelligence. Speaking on condition of anonymity, they said they did not know how much hexaflouride was made and when the testing resumed.

The move — coupled with revelations Tuesday that Iran had restarted building centrifuges — heightened concern that Iran was moving toward full uranium enrichment, despite pledges not to do so in the interest of building international goodwill.

In Washington, State Department spokesman Adam Ereli said Iran had launched a direct challenge to the IAEA's call to suspend all enrichment-related and reprocessing activities.

"It certainly raises questions about other commitments Iran has made concerning its nuclear program," he said.

Iran dismissed accusations it is interested in making nuclear weapons (search), insisting its main interest in nuclear power is to generate electricity. But one of the diplomats said the news was part of a pattern of recent revelations showing Iran to be more interested in pressing ahead with suspect nuclear activities than working to dispel worldwide concerns.

IAEA (search) officials had no comment about the revelations, which came only a day after diplomats disclosed that Tehran had resumed building centrifuges.

That move alarmed France, Germany and Britain, which have been seeking a negotiated resolution with Iran, and was likely to move them closer to the United States, which insists Tehran wants to make nuclear weapons.

German Foreign Minister Joschka Fischer expressed "great concern" Wednesday over the reported restart of centrifuge construction, and cautioned Iran against making a "miscalculation."

Separately, diplomats citing an intelligence report also told AP that Iran is trying to make or buy deuterium gas, a substance that has peaceful uses but can also be used to boost the power of a nuclear explosion.

One of the diplomats said Iranian agents were trying to buy the gas on the Russian market and had plans to manufacture it domestically. But Russia's Foreign Ministry said that Moscow "does not plan to make such deliveries."

Beyond increasing the punch of nuclear warheads, the gas also can be used as a coolant for heavy water nuclear reactors. Iran is building a heavy water facility and one of the diplomats said Iran was likely looking for the substance "to get the reactor going."

Another diplomat familiar with Iran's nuclear activities also suggested Iran's interest was linked to its research reactor. Deuterium can only be used to boost nuclear explosions if combined with tritium, and there was no evidence Iran was trying to acquire that substance, he said.

The reactor itself is one of several projects that have increased suspicions about Tehran's nuclear aims.

Heavy water can be used to make plutonium. Iran says it needs the plutonium from the research reactor for isotopes in medical research but plutonium — like enriched uranium — can also be used to make nuclear warheads.

For the past year, the IAEA has been carrying out stringent inspections of Iranian facilities, uncovering evidence that strengthened suspicions about Tehran's nuclear ambitions. In June, the IAEA's Board of Governors rebuked Tehran in a sharply phrased resolution indicating it felt too many unanswered questions remained.

British, French and German officials will meet with Iranian representatives in the next few days to try to gain a renewed commitment that Tehran will not enrich uranium — an unlikely prospect, considering the recent developments.

Most of the IAEA's concerns about the Islamic Republic's nuclear program focus on traces of highly enriched uranium found at several sites and the extent and nature of work on the advanced P-2 centrifuge.

Iran has grudgingly acknowledged working with the P-2 but said its activities were purely experimental. It says the minute amounts of enriched uranium were from equipment bought on the nuclear black market.

IAEA chief Mohamed ElBaradei has indirectly questioned such assertions.