VIENNA, Austria – The U.N. nuclear watchdog expressed “serious concern” yesterday over Iran’s resumption of activities that could lead to an atomic bomb, and diplomats said Tehran has a September 3 deadline to stop or face another possible referral to the Security Council.

Iran, showing the defiance it has increasingly displayed since its new president was inaugurated last weekend, responded with indignation. Tehran’s chief delegate here vowed that Iran would become a nuclear-fuel producer and supplier within a decade.

“This resolution is political,” an Iranian Foreign Ministry spokesman, Hamid Reza Asefi, said, according to the state-run news agency. “It comes from American pressure. … It lacks any legal or logical basis and is unacceptable.”

The topic of the International Atomic Energy Agency resolution, adopted by consensus by its 35-nation board, was Iran’s move Wednesday to reopen its uranium conversion plant in the mountains outside the southern city of Isfahan. With the plant now working at full force, Iran’s hard-liners are pushing President Ahmadinejad to ignore European warnings and resume uranium enrichment.

A member of Iran’s powerful Expediency Council, Mohammad Javad Larijani, said the transfer of power to Mr. Ahmadinejad has given the country an opportunity to change the rules of the game. He called France, Germany, and Britain – the countries negotiating with Iran – “three international savages” and said any debate over enrichment is “shameful.”

Starting up the enrichment facility, a plant built mainly underground outside the city of Natanz to protect it from airstrikes, would heighten tensions with Europe and America. Enrichment is the final step in uranium development, producing either fuel for a nuclear reactor for electricity or material for a nuclear bomb.

Iran denies it seeks to develop nuclear weapons and says its program is only for peaceful purposes. But Tehran insists it has the right under the Nuclear Nonproliferation Treaty to develop the full fuel cycle, including enrichment.

President Bush, meeting at his Texas ranch with his foreign-policy team, welcomed the nuclear agency’s warning to Tehran. He also indicated that Mr. Ahmadinejad will receive an American visa to attend an annual U.N. gathering next month in New York.

Britain’s Foreign Office said the IAEA resolution “sends a clear message to Iran of what it must do. We still believe there is a non-confrontational way forward if Iran wants to take it.”

In Vienna, the nuclear agency asked IAEA’s chief, Mohamed ElBaradei, to deliver a report on Iran’s implementation of nuclear safeguards by September 3. Diplomats made clear that insufficient progress by that date could mean the board would consider referring Iran to the U.N. Security Council, which has the power to impose sanctions. The diplomats spoke on condition of anonymity because they were not authorized by their governments to discuss the issue.

Yesterday’s resolution did not mention the Security Council, given concerns such a move could backfire by hardening Iran’s position. Iran had said it would rather endure sanctions than back down.

Security Council diplomats in New York say the IAEA may also be wary of referring Iran to the council because there is a real risk the body would not agree to sanctions. China, for example, has said it opposes bringing the issue before the council, and could use its veto power to block a resolution punishing Iran.

The board’s next scheduled meeting is September 19, but members can call emergency meetings at any time. This week’s meetings were called by France, Germany, and Britain after Iran announced it planned to resume uranium conversion.

Iran had suspended that process and the subsequent enrichment process under an agreement with the three E.U. countries.

Tehran saw the text adopted yesterday as unacceptable because it would bar it from enrichment and other related activities that are allowed under the Nuclear Nonproliferation Treaty, or NPT, the country’s chief IAEA delegate, Sirus Nasseri, said.

Mr. ElBaradei said he was “very encouraged” by statements from the European Union and Iran that the talks would continue.

E.U. envoys said the burden was now on Iran to keep talks alive.

“A breakdown will be a matter of regret to the E.U., because the E.U. hoped that it could persuade Iran to take measures that might lead to a restoration of international confidence in Iran’s nuclear intentions,” the statement said. “But the E.U. is confident that another way of making possible the necessary restoration of confidence in Iran’s nuclear intentions can be found.