VIENNA (Reuters) - A U.S. think tank said Iran might be cleaning up its Parchin military site, where some countries suspect experiments may have taken place in a possible atomic weapons program, but Iran denied this on Thursday.
The U.S.-based Institute for Science and International Security (ISIS) pointed to satellite images showing vehicles and container-like objects being moved at Parchin. Iran said this was part of road works in the area.
ISIS said the images were taken after Iran signed a major deal with world powers to curb its nuclear activities in exchange for sanctions relief.
Such activity might complicate the work of the U.N.'s International Atomic Energy Agency (IAEA), whose job it is to judge whether Iran's past atomic activity had any military dimensions, including through access to Parchin.
"This renewed activity occurring after the signing of the (July 14 deal) raises obvious concerns that Iran is conducting further sanitization efforts to defeat IAEA verification," the Washington-based think tank said in a report.
"This renewed activity may be a last ditch effort to try to ensure that no incriminating evidence will be found," said ISIS, which was founded by a former IAEA nuclear inspector to bring scientific expertise to debates about nuclear weapons.
Iran's mission to the United Nations in New York issued a statement saying it "strongly denies and rejects the baseless claims about the so-called clean-up operations in the Parchin Military Complex".
It said there was construction work at Parchin to repair a road and denounced "the extensive vicious campaign at work ... to poison the positive environment at the global level."
"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 added.
U.S. State Department spokesman Mark Toner told reporters on Thursday he could not confirm the group's findings but that any clean up effort would be "cause for concern." He said the United States is confident it knows what has occurred at Parchin as well as its 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.
White House spokesman Josh Earnest separately played down any such Iranian efforts, although he said at a daily briefing he could not discuss any specific intelligence matters.
According to data given to the IAEA by some member states, Parchin might have housed hydrodynamic experiments to assess how specific materials react under high pressure, such as in a nuclear blast.
A diplomat familiar with the Iran file said that the images showed the difficulty of implementing the July deal. "Old habits die hard," he said, referring to Iran, which in the past had failed to declare some of its nuclear activities to the IAEA.
The Vienna-based agency, which had no immediate comment on the ISIS report, said in its latest extensive Iran report that it continued to observe vehicles and construction material at Parchin and that activities there since 2012 are likely to have undermined its ability to conduct verification.
As a precondition to full sanctions relief, Tehran must provide sufficient information to the IAEA by Oct. 15 to allow it to prepare a final report on its past nuclear program.
The Obama administration is defending the nuclear deal with Iran against strong opposition by some lawmakers who also criticize the IAEA for not releasing its own agreement with Iran to resolve concerns about Iran's nuclear past.
"I explained that my legal obligation is to protect safeguards confidentiality and this is an essential element of the international safeguards regime," IAEA head Yukiya Amano said after meeting U.S. politicians this week.
Reporting by Shadia Nasralla in Vienna, Louis Charbonneau at the United Nations, Parisa Hafezi in Ankara and Lesley Wroughton, Susan Heavey and Doina Chiacu in Washington; Editing by Tom Heneghan and Grant McCool