An IAEA report last November found Iran had built a large containment vessel in 2000 at the site in which to conduct high-explosive tests that the U.N. agency said were “strong indicators of possible (nuclear) weapon development”.

“A building was constructed at that time around a large cylindrical object … A large earth berm was subsequently constructed between the building containing the cylinder and a neighboring building, indicating the probable use of high explosives in the chamber.”

The IAEA said it had obtained satellite images that were consistent with this information. The vessel was designed to contain the detonation of up to 70 kg of high explosives.

Western diplomats say they suspect Iran may be cleaning the location to remove incriminating evidence before inspectors can go there. A U.S. security institute said last week satellite imagery showed activity there which it said raised concern that Iran may be “washing” the building the IAEA wants to see.