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

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

Iran has described its cooperation with the IAEA probe of its alleged nuclear weapons experiments as positive, suggesting it was providing information requested by agency officials. Indirectly disagreeing, the agency also said that Iran continued to deny such activities, dismissing evidence to the contrary submitted for its perusal as misleading or false.

The findings were part of a restricted agency report forwarded to the U.N. Security Council and to the 35 board members of the IAEA that was obtained by The Associated Press.

The nine-page report also noted that Tehran remains defiant of U.N. Security Council demands to stop uranium enrichment.

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

In announcing major progress in Iran’s push for nuclear power, President Ahmadinejad said last month that Iranian scientists were putting 6,000 new uranium enriching centrifuges into place and testing a new type that works five times faster.

The report noted that Iran now had only 3,500 centrifuges and specified that the few advanced machines actually running were only in a testing phase.

Uranium can be used as nuclear fuel or as the fissile core of warheads, depending on the degree of enrichment. Iran insists it has a right to the activity under the Nuclear Nonproliferation Treaty and says it wants only to generate power.

But addressing whether Iran was complying with agency requests, the report said that “Iran has not provided the Agency with all the information, access to documents and access to individuals necessary to support Iran’s statements” that its activities were purely peaceful in intent.

“The Agency is of the view that Iran may have additional information, in particular on high explosives testing and missile related activities which … Iran should share with the agency,” said the report. It was referring to two alleged sets of tests that agency officials say could be linked to a nuclear program.

Iran already rejected evidence provided by America and other IAEA board members on alleged weapons programs in February, but then promised to revisit the issue ahead of the agency’s next board meeting in a week.

Iran’s nuclear programs have been under IAEA investigation since 2003, after a dissident group revealed the existence of a clandestine enrichment program.

A senior American envoy suggested the report was a strong indictment of Iran’s defiance of the international community’s efforts to seek answers about trouble 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 resolution,” the chief American delegate to the IAEA, Gregory L. Schulte, said. He described parts of the report as a “direct rebuttal” of Iranian claims that all nuclear questions had been answered.