Iran on Monday began enriching uranium up to 20 per cent at an underground facility.

The decision comes after parliament passed a bill, later approved by a constitutional watchdog, aimed at hiking enrichment to pressure Europe into providing sanctions relief.

As negotiators were grasping for a landmark nuclear deal, they hit a roadblock over Iran's once-secret enrichment site at Fordo, a fortified bunker buried deep in a mountainside - potentially impervious to U.S. or Israeli airstrikes.