A new International Atomic Energy Agency report said Monday that Iran has repeatedly blocked a U.N. investigation into allegations it tried to make nuclear arms and the probe is now deadlocked.

Since its last report in May, "the agency ... has not been able to make any substantive progress," said a copy of the IAEA report obtained by The Associated Press. It called the impasse a matter of "serious concern."

The IAEA report was released to the 35-nation IAEA board and the U.N. Security Council, which has already imposed three sets of sanctions on Iran over its nuclear defiance.

The document said Iran has now amassed a third of the amount of enriched uranium it could reprocess into the material for the fissile core of a nuclear weapon should it choose to do so. But U.N officials familiar with the report emphasized that Iran — whose known nuclear programs are under IAEA supervision — has shown no indication it wanted to go that route.

"We''ve arrived at a gridlock," said a senior U.N. official, describing the document as "a progress report without progress." He demanded anonymity because he was not authorized to discuss the restricted report.