New satellite photographs appear to show that unusual activity has taken place in recent weeks at the Parchin site outside Tehran, according to the Institute for Science and International Security (ISIS).

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.