A large stream of water pictured flowing from the building, said to feature an   “explosive chamber” for testing, “raises concerns that Iran may have been   washing inside the building”, according to the group.

A series of unidentified items were also shown lined up outside the building   in the April 9 photographs. Pictures taken in March and in July 2011 showed   neither, suggesting unusual recent developments.

David Albright, ISIS’s founder, said its finding backed up concerns at the   International Atomic Energy Agency (IAEA), which has been denied access to   the site since 2005, over recent “activity” there.

“Iran should immediately allow IAEA inspectors into the Parchin site and allow access to this specific building,” said Mr Albright, a former inspector. “It should also explain the purpose of the activities”.

Yukiya Amano, the IAEA Director General, said earlier this year that he had   “information” that “activity is ongoing” at the site, which is at the centre   of the controversy over Iran‘s   nuclear scheme.

The IAEA believes that a large explosives containment chamber was installed at   the complex in 2000 for explosives experiments that are “strong indicators   of possible weapons development.”

Tehran insists its programme remains devoted to peaceful technology and that   concerns to the contrary are based on “fake evidence”. Leading powers have   demanded that Iran agree to abandon its production of highly-enriched   uranium at talks in Iraq later this month.

Foreign minister Ali Akbar Salehi said last month his country would resolve   questions on the programme “quickly and simply” but Iran has shown no   willingness to allow inspectors into Parchin.

A spokesman for the State Department declined to comment. Spokesmen for the   IAEA were not available for comment.