Iran said allegations about activity at its Parchin military site were "lies" spread by critics of its historic nuclear deal with major powers last month.
The Institute for Science and International Security (ISIS) on August 6 pointed to satellite images showing a bulldozer or steamroller, new structures, and container-like objects being moved at Parchin.
Iran said the activity was part of road works in the area, but the U.S. think tank questioned that explanation on August 7.
Iranian Foreign Minister Mohammad Javad Zarif was quoted on August 8 as saying by the IRNA state news agency that critics of the nuclear deal "have spread these lies before."
"Their goal is to damage the agreement," he said.
The United States and Israel suspect Tehran experimented with developing nuclear weapons at the Parchin site.