Damaged or destroyed - how much does leaked US report on Iran's nuclear sites tell us?

The site of Fordo is probably the most spied-on place on the planet.

Western intelligence first went public in 2009 that it was home to a secret nuclear facility and now understanding the damage done by US strikes will be vital in determining where the conflict goes next.

A leaked Defence Intelligence Agency (DIA) assessment has suggested the core components of Iran's nuclear programme have not been destroyed and the strikes only set back Iran's efforts by months rather than years.

But that is only an initial assessment and labelled as "low confidence" – the tag comes because it is early days in trying to understand what happened at a place which is deliberately hidden from prying eyes.

The DIA is the Pentagon's own agency which specialises on military intelligence to support operations. It collects large amounts of technical intelligence but is distinct from other agencies like the CIA.

"Final battle damage will take some time," Chairman of the Joint Chiefs General Dan Caine said in the immediate aftermath. But what does it mean to destroy or damage the programme and how do you find out?

Satellite images of holes and dust reveal little about what really happened underground. And they do not suggest massive subsidence or a cave-in of the mountain.