Iran has shrouded a building that the U.N. nuclear agency suspects was used for secret work on atomic weapons, meaning spy satellites can no longer monitor Tehran's alleged efforts to clean up the site, diplomats told The Associated Press on Friday.

For months, satellite images have recorded what the International Atomic Energy Agency suspects is an attempt to sanitize the site. At the same time, Iran has repeatedly rebuffed agency efforts for access — and did so again Friday. Talks between the two sides ended without an agreement that would have allowed IAEA experts to visit the site at the Parchin military complex southeast of Tehran.

The diplomats said the main building is now covered with what appears to be plastic sheeting, shielding any activity there from the outside world and effectively shutting down the IAEA's only way of monitoring the site with its eyes in the sky through spy and commercial satellite imagery.