The International Atomic Energy Agency (IAEA) says Iran is broadly complying with agreements on its nuclear program.
In a confidential report on August 27 obtained by Western news agencies, the UN agency says Iran's current level of uranium enrichment, nuclear research, and development is in line with its declarations.
It said, however, that it recently observed through satellite imagery the presence of vehicles, equipment, and probable construction materials at the military base of Parchin.
"The activities that have taken place at this location since February 2012 are likely to have undermined the agency's ability to conduct effective verification," the report reads.
"Iran doesn't need to ask for the IAEA's permission to do construction work on its sites," Reza Najafi, Iran's envoy to the agency, was quoted as saying.
Western intelligence agencies allege Iran used the site for experiments related to the development of nuclear weapons, which Tehran denies.
The IAEA's report covers Iran's nuclear program in the run-up to the July 14 nuclear agreement between Tehran and world powers and its first steps toward implementing that accord.