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.

Meanwhile, explosions and fires were reported in Tehran and Shiraz.

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.