Tehran may have used the delay in the US's potential strike to make the beneath-mountain facility nearly untouchable.

The activity comes at a time of heightened tensions as talks between Iranian and US officials continue over Iran's nuclear programme.

Since the US bombed Iran's key nuclear sites last year, analysts say work has increased at a secret underground facility that could be used to restart Tehran's nuclear program.

The notion that Iran only has these various ballistic missiles for defense or maintaining deterrence is contradicted by many of its actions since 2015.