Defying recent requests from the U.N. nuclear watchdog agency, the Iranian government said Tuesday it had begun a new round of nuclear experiments, intensifying concerns of U.N. weapons inspectors and the U.S. government that the Islamic republic has plans to develop nuclear weapons.

The head of Iran's nuclear energy program, Gholamreza Aghazadeh, told reporters in Vienna on Tuesday that scientists were following through on plans to convert nearly 40 tons of raw uranium into a state suitable for enrichment.

Two years ago, Iran enriched low levels of uranium that could be used in a civilian energy program. But once the enrichment technique is mastered, it could be used to produce weapons-grade uranium.

U.S. officials said they viewed the move as proof that Iran has no interest in abandoning its nuclear ambitions or respecting decisions made by the board of the International Atomic Energy Agency last week.

"This clearly indicates Iran is continuing its unrelenting march toward a nuclear weapons capability," said J. Adam Ereli, a State Department spokesman. "Iran knows what it must do to alleviate concerns, but so far it appears determined to ignore the IAEA and proceed with its nuclear weapons program."

IAEA inspectors have not found evidence of a weapons program. But Iranian experiments, concealed for years from the outside world, have fueled suspicion that the Tehran government is determined to build a bomb.

Last Saturday, the United States and other members of the IAEA board called on the Iranian government to suspend enrichment-related activities and provide the agency with a complete history of its nuclear program.

The resolution noted, "with serious concern," that Iran had not "heeded repeated calls from the Board to suspend, as a confidence building measure, all enrichment-related and reprocessing activities." Iran's intention to process the 40 tons of raw uranium into uranium hexaflouride gas "would run counter" to early requests to forgo the work, the resolution stated.

But the IAEA board also noted recent improvements in Iran's cooperation with agency inspectors and recognized its right to develop a peaceful energy source. Iran's president, Mohammad Khatami, indicated in a speech in Tehran on Tuesday that his country was prepared to retain that right at great cost.

"We are determined to obtain peaceful atomic technology, even if it brings an end to international supervision," Khatami was quoted as saying. Iran notified the IAEA weeks ago that it planned to proceed with the conversion experiments, and the agency said it would monitor the work.

It is permissible under the nuclear Non-Proliferation Treaty to process uranium for an energy program, as long as the IAEA is informed of the work. But the IAEA has noted that Iran's energy program is at an early stage and as far as 10 years away from needing converted uranium to operate.

The IAEA director general, Mohamed ElBaradei, said that conducting such work now, while international pressure is high, would hurt Iran's credibility and could jeopardize important gains it had made in recent months with IAEA inspectors.

"We have repeated calls on the Iranians to suspend this kind of testing in order for it to build international confidence," said Melissa Fleming, the IAEA spokeswoman.

A secret, 18-year effort by Iran to obtain nuclear technology and know-how was exposed by an opposition group two years ago. Since then, IAEA inspectors investigating Iran's program have uncovered equipment purchased on a black market run by Abdul Qadeer Khan, who guided the development of Pakistan's nuclear bomb. But ElBaradei said last week that there was no evidence of an Iranian nuclear weapons program. Iranian authorities insist their nuclear program is intended solely to generate electricity.

"I'm not sure we are facing an imminent threat," ElBaradei said in an interview with CNN on Sunday. But he said it was clear Iran was acquiring, or has already acquired, "a capability to produce the material that can be used for nuclear weapons should they decide to do that."

Iran has promised several times in the past year to halt enrichment work but then reneged on aspects of those pledges in response to IAEA decisions. Last week, Iranian officials in Vienna said the country would continue a suspension on enrichment but did not specify whether it would halt the processing of uranium hexaflouride gas.

The announcement by Aghazadeh, who is one of Iran's vice presidents as well as head of the country's nuclear energy program, dispelled any doubt on the issue. He said Iran had begun work with some of the 40 tons marked for conversion.

"The tests have been successful, but these tests have to be continued using the rest of the material," Aghazadeh said in Vienna, where he was attending an IAEA conference.

Gen. Nasser Mohammadifar, Iran's army commander, confers with President Mohammad Khatami during a parade in Tehran commemorating the start of the Iran-Iraq war. Khatami said Iran was "determined to obtain peaceful atomic technology, even if it brings an end to international supervision."