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.

WHAT IRAN COULD DO TO. RESPOND. FOR 20 YEARS, U.S. PRESIDENTS HAVE BEEN SAYING THAT IRAN CANNOT BE ALLOWED TO OBTAIN A NUCLEAR WEAPON. THE CONSENSUS HAS BEEN CLEAR ACROSS ADMINISTRATIONS AND PARTY ...

We explore why Tehran has responded so meekly to recent attacks on its top officials and close allies. By David Leonhardt It has become a mystery in the current Middle East conflict: Why has Iran ...