A U.S. think-tank said Iran might be cleaning up its Parchin military site, where the United States and Israel suspect Iran experimented with developing nuclear weapons, but Tehran vehemently denied this.
The Institute for Science and International Security August 6 pointed to satellite images showing a bulldozer or steamroller, new structures, and container-like objects being moved at Parchin -- activity that Iran said was part of road works in the area.
The group, which was founded by a former United Nations nuclear inspector to bring scientific expertise to debates about nuclear weapons, said the images were taken after Iran signed a major deal July 14 with world powers to curb its nuclear activities in exchange for sanctions relief.
"This renewed activity...raises obvious concerns that Iran is conducting further sanitization efforts to defeat...verification" of the deal, the nonpartisan think-tank said.
"This renewed activity may be a last ditch effort to try to ensure that no incriminating evidence will be found."
The sighting comes only a day after top U.S. senators raised concerns about whether the deal would allow UN inspectors the access they need to inspect the Parchin site south of Tehran.
More broadly, many legislators also have questioned whether the deal is air-tight enough to prevent Iran from cheating and hiding or cleaning up nuclear-arms activities.
The think-tank's discovery also raises major headaches for the UN's International Atomic Energy Agency, whose job it is to monitor Iran's compliance with restrictions on its nuclear activities, as well as judge whether Iran's past atomic activity had any military dimensions.
The Vienna-based UN agency was expected to visit Parchin soon and take environmental samples to determine the presence of radioactive materials.
The agency had no immediate comment on the think-tank report. But it has repeatedly cited previous evidence of possible attempts to sanitize the Parchin site
The agency's latest report on Iran said it had observed vehicles and construction material at Parchin, and that activities there since 2012 are likely to have undermined its ability to conduct verification.
U.S. senators said this week that the agency was not able to assure them that it could gain physical access to Parchin to investigate charges from the United States and Israel that Iran tried to develop weapons there.
In Tehran, the foreign ministry called the think-tank report "baseless [and] ridiculous," saying construction at the site is linked to road repairs. It noted that a road near the Mamloo Dam, which is near Parchin, required repairs using heavy bulldozers and other heavy construction machinery.
White House spokesman Josh Earnest acknowledged that Tehran might be trying to cover up weapons work at Parchin, but downplayed the significance of the finding.
"For more than a decade, the Iranians had gone to great lengths to try to cover it up," he said. "We're not particularly concerned that, over the course of the next couple of weeks, that they're going to succeed in covering up something that they haven't been able to cover up over the last decade."
Iran's mission to the UN called the think-tank's findings part of an "extensive vicious campaign at work...to poison the positive environment."
"The Islamic Republic of Iran...has never had any military nuclear activity and has never been engaged in any unconventional act that would need a hasty cover-up," it said.
U.S. State Department spokesman Mark Toner said he could not confirm the group's findings, but any clean-up effort would be "cause for concern."
He said the United States is confident it knows what has occurred at Parchin and has the ability to detect previous nuclear activity at any Iranian site.
"You can't cover up past nuclear activity very easily - it lasts for decades, even longer," Toner said.
According to data given to the UN by some member states, Parchin might have housed hydrodynamic experiments to assess how specific materials react under high pressure, such as a nuclear blast.
A diplomat familiar with the Iran file said that the recent satellite images showed the difficulty of implementing the nuclear deal.
"Old habits die hard," he said, referring to Iran's past failure to declare some of its nuclear activities to the UN watchdog.
As a precondition to getting full sanctions relief, Tehran must provide sufficient information to the agency by October 15 to allow it to prepare a final report on Iran's past nuclear program.