In a briefing Thursday, State Department spokesperson John Kirby said that the Parchin base in Iran is not a nuclear site.
"It’s important to remember that when you’re talking about a site like Parchin you’re talking about a conventional military site, not a nuclear site," he said.
Kirby added that Parchin would not face International Atomic Energy Agency (IAEA) "or other restrictions" on construction, "were they to occur."
When asked why the Parchin base was included in the report if it was not a nuclear base, he referred reporters to the IAEA.
The IAEA report states that Iran may have extended its Parchin facility since May.
In addition, data given to the IAEA by member states suggests that the site might have been used for hydrodynamic experiments related to nuclear bomb detonations, according to Reuters:
Iran appears to have built an extension to part of its Parchin military site since May, the U.N. nuclear watchdog said in a report on Thursday, as part of its inquiry into possible military dimensions of Tehran's past nuclear activity.
A resolution of the International Atomic Energy Agency's Parchin file, which includes a demand for fresh IAEA access to the site, is a symbolically important issue that could help make or break Tehran's July 14 nuclear deal with six world powers.
The confidential IAEA report, obtained by Reuters, said:
"Since (our) previous report (in May), at a particular location at the Parchin site, the agency has continued to observe, through satellite imagery, the presence of vehicles, equipment, and probable construction materials. In addition, a small extension to an existing building" appeared to have been built.
The changes were first observed last month, a senior diplomat familiar with the Iran file said.
The IAEA says any activities Iran has undertaken at Parchin since U.N. inspectors last visited in 2005 could jeopardize its ability to verify Western intelligence suggesting Tehran carried out tests there relevant to nuclear bomb detonations more than a decade ago. Iran has dismissed the intelligence as "fabricated".
Under a "road map" accord Iran reached with the IAEA parallel to its groundbreaking settlement with the global powers, it is required to give the Vienna-based watchdog enough information about its past nuclear activity to allow it to write a report on the long vexed issue by year-end.
The IAEA has been regularly reporting on nuclear activity in Iran since 2003.