A defiant Iran did not turn up for a promised meeting Thursday with the head of the U.N. nuclear watchdog agency to discuss the country's suspect nuclear program, agency officials said.

A high-ranking Iranian delegation was supposed to meet International Atomic Energy Agency head Mohamed ElBaradei on Thursday to provide him with details of its plans to move closer to uranium enrichment, a process which can produce both fuel for nuclear energy or nuclear weapons.

"The meeting never took place," said IAEA spokeswoman Melissa Fleming. She said ElBaradei was "still seeking clarification" of what Tehran would do Monday, when it says it will resume some work related to enrichment.

Iran told the IAEA on Tuesday it planned to resume nuclear fuel research after a 2 1/2-year hiatus, posing a fresh challenge to Western nations concerned that Tehran is trying to build an atomic weapon.

A diplomat accredited to the IAEA said the agency appeared resigned to not getting the details it had asked for before the Iranians resume the enrichment work. He cited ElBaradei as saying he did not expect the Iranian delegation to ask for a new appointment before flying back to Tehran.

Iranian representatives already rebuffed ElBaradei on Wednesday when he asked for clarification of their plans. But they said then they would be more forthcoming Thursday on what they describe as the resumption of research and development of uranium enrichment.

The United States has warned Iran against pursuing new nuclear research. Washington said the international community will have to consider additional measures to constrain Iran's nuclear ambitions if the country takes any further steps related to uranium enrichment. That appeared to be an allusion to having Tehran referred to the U.N. Security Council.

The path for such a move was cleared late last year, when the IAEA's 35-nation board found Iran in noncompliance of the Nuclear Nonproliferation Treaty for keeping its uranium enrichment program secret for decades and conducting other work that could be used for a nuclear weapons program.

The European Union, with U.S. backing, then decided to give diplomacy another try in efforts to gain more international support for their stance.

Iran says it is interested in enrichment to make nuclear fuel, but the United States and an increasing number of other nations suspect Tehran wants the technology to make weapons-grade uranium for nuclear warheads.

Tehran says it will not actually begin enrichment Monday. But even the restarting of equipment testing would be viewed as the latest in a series of steps toward fully reviving the program in violation of Tehran's pledge for a full freeze on all of its aspects.

A diplomat close to the agency said that the Iranians appeared taken aback by the firmness of ElBaradei's demands Wednesday for more cooperation in his agency's probe of Tehran's nuclear activities. The diplomats who spoke to The Associated Press asked for anonymity because the meeting was private.

ElBaradei's demands and clear criticism of Iran's plans to move closer to enrichment was what apparently led the Iranians to not show for Thursday's meeting, the diplomat said.

Iran has come under heavy international pressure to abandon its program to produce fuel for its Russian-built nuclear reactor that is due to come online this year and for its future nuclear power plants.

But it has vowed never to give up the right to enrichment, which it says it has a right to under international law.

While refusing to permanently renounce uranium enrichment, Iran suspended many aspects of its nuclear fuel program in 2003 as a goodwill gesture during negotiations with the big three European powers — Britain, France and Germany.

The talks collapsed in August after Iran resumed uranium reprocessing activities, a step before enrichment, at its Uranium Conversion Facility in Isfahan, central Iran.

While the two sides resumed dialogue last month, talks have so far failed to resolve the dispute. More talks are scheduled for later this month.