UNITED NATIONS: A prominent United States think tank on Friday questioned Iran's explanation for activity at its Parchin military site visible in satellite imagery, saying the movement of vehicles did not appear related to road work.

The US-based Institute for Science and International Security (ISIS) said this week that Iran might be sanitising its Parchin military site, where some countries suspect experiments may have taken place in a possible atomic weapons program. Iran denied it, saying it was part of road works near the Mamloo Dam.

The think-tank issued a fresh analysis on Friday disputing Iran's story.

"Commercial satellite imagery does not support the Iranian explanation," the think tank said in a statement. "ISIS analysed commercially available satellite imagery taken on July 12, 19, and 26, 2015 but did not find any visible signatures related to road work on the road near the dam."

It said it would make little sense for Iran to "park vehicles three kilometres south of the dam and at the one site that would create intense concern and suspicion about Iran's intentions to comply with the recently negotiated (deal)." A spokesman for Iran's United Nations mission in New York said Tehran stood by its statement from Thursday and had nothing to add.

Parchin is a site the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), requested access to as part of a landmark July 14 nuclear deal between Tehran and six major powers. ISIS suggested Iran could be engaged in cleanup work before IAEA inspectors arrive.

Attempts to sanitise the site could complicate the work of the IAEA, whose job it is to judge whether Iran's past atomic activity was linked to developing weapons, including through access to Parchin.

"There is no support in the imagery for the Iranian explanation," ISIS's new analysis said. "Iran's explanation appears to be that the vehicles at the suspect site were there inadvertently, but this explanation strains believability."

US State Department spokesman Mark Toner said on Thursday he could not confirm the findings but that any cleanup effort would be of concern. The ISIS findings come as the Obama administration is struggling to convince skeptics in Congress to support the deal.

Under the July 14 nuclear deal, most sanctions on Iran will be lifted once the IAEA verifies Iranian compliance with the terms of the agreement, including strict limits on Iranian nuclear activities and answering questions about its past atomic work. Iran says it has never sought atomic weapons.