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

The tone of the language suggesting Tehran continues to stonewall the U.N. nuclear monitor revealed 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 media.

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