A top Iranian official on Sunday abruptly canceled a meeting with the head of the International Atomic Energy Agency, dealing a blow to IAEA efforts to investigate alleged attempts by Tehran to make nuclear arms, an agency official said.

The IAEA official, confirming Iranian media reports that Monday's planned meeting was off, told The Associated Press that no reason had been given.

But a senior diplomat had told the AP that IAEA head Mohamed ElBaradei likely planned to use the meeting with Gholam Reza Aghazadeh, the head of Iran's nuclear program, to renew a request for more information on allegations Tehran had tried to make atomic arms.

Both the official and the diplomat demanded anonymity because they were not authorized to comment by name on the Iranian nuclear issue.

The diplomat, who follows IAEA attempts to clear up suspicions about Iran's nuclear activities, said the meeting also was likely to have focused on Iran's latest show of defiance of U.N. Security Council demands to suspend uranium enrichment.

Last week Iranian President Mahmoud Ahmadinejad announced that his nation was installing thousands of new uranium-enriching centrifuges and testing a much faster version of the device.

Ahmadinejad said scientists were putting 6,000 new centrifuges into place, about twice the current number, and testing a new type that works five times faster.

That would represent a major expansion of uranium enrichment — a process that can produce either fuel for a nuclear reactor or material for a warhead.

Still, others suggested the claims may be exaggerated.

U.S. Secretary of State Condoleezza Rice cautioned the claim could not be immediately substantiated, and diplomats close to the IAEA said Iran has exaggerated its progress and experienced problems operating the 3,000 centrifuges already in place. One diplomat said Ahmadinejad's claims of a more advanced centrifuge appeared to allude to a type known as the IR-2, which the agency and Iran said months ago that Iran had begun testing.

The IR-2 is believed to be two-to-three times faster than the centrifuges currently in use, and his claim that the new machine was five times as quick added to the diplomats' skepticism.

In the enrichment process, uranium gas is pumped into a series of centrifuges called "cascades." The gas is spun at supersonic speeds to remove impurities. Enriching at a low level produces nuclear fuel, but at a higher level it can produce the material for a warhead. Iran says it is only interested in the process to generate nuclear power and plans to move toward large-scale uranium enrichment that ultimately will involve 54,000 centrifuges.

The canceled meeting was also consider the first test of whether Iran will truly continue to stonewall the IAEA, the U.N. nuclear watchdog, in its attempt to investigate the alleged military programs after saying earlier this year that such allegations were fabricated.

Iran has denied ever trying to make such weapons. But U.S. intelligence agencies say Tehran experimented with such programs until 2003, and other countries believe it continued past that date.

Suspected weapons-related work outlined in a February presentation to the IAEA's 35-nation board and IAEA reports preceding it include:

• Uranium conversion linked to high explosives testing and designs of a missile re-entry vehicle, all apparently interconnected through involvement of officials and institutions.

• Procurement of so-called "dual use" equipment and experiments that could be used in both civilian and military nuclear programs.

• Iran's possession of a 15-page document outlining how to form uranium metal into the shape of a warhead.

Iran is under three sets of Security Council sanctions for its refusal to suspend enrichment and meet other council demands designed to ease fears its civilian nuclear program is a cover for attempts to make atomic arms.