VIENNA — Iran may be withholding information needed to establish whether it tried to make nuclear arms, the International Atomic Energy Agency said yesterday in an unusually strongly worded report.

The tone of the language suggesting Tehran continues to stonewall the U.N. nuclear monitor showed a glimpse of the frustration felt by agency investigators stymied in their attempts to gain full answers to suspicious aspects of Iran’s past nuclear activities.

A senior U.N. official familiar with the investigation into Iran’s nuclear program said none of the dozens of agency reports issued in that context had ever been as plain spoken in calling Tehran to task for not being forthright. He agreed to discuss the report only if granted anonymity because he was not authorized to comment to the press.

Iran has described its cooperation with the agency’s probe as positive, suggesting it was providing information requested by agency officials.

Iran’s chief delegate to the IAEA, Ali Ashgar Soltanieh, said as much again yesterday, telling the Associated Press that the report described “the peaceful nature of our nuclear actions.”

“The Americans failed … in shameful attempts” to co-opt the agency into delivering anti-Iranian findings, he said.

He noted a paragraph in the report saying that agency experts had been given access to all declared nuclear material in Iran and verified that all of it was accounted for.

But his American counterpart, Gregory Schulte, suggested the report was a strong indictment of Iran’s defiance of the international community’s efforts to get answers about troubling parts of its nuclear program, noting it “details a long list of questions that Iran has failed to answer.”

“At the same time that Iran is stonewalling its inspectors, it’s moving forward in developing its enrichment capability in violation of Security Council resolutions,” Mr. Schulte told the AP.

He described parts of the report as a “direct rebuttal” of Iranian claims that all nuclear questions had been answered.

American intelligence says Iran stopped work on nuclear weapons in 2003, but some other nations believe such activities continued past that date. The report noted Iran continued to deny such allegations.

Obtained by the AP, the restricted report forwarded to the U.N. Security Council and to the 35 board members of the IAEA said Iran remains defiant of the council’s demands that it suspend uranium enrichment.

Shrugging off three sets of council sanctions, Iran has expanded its operational centrifuges — machines that churn out enriched uranium — by about 500 since the last IAEA report, in February, the new report said.