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.