The International Atomic Energy Agency has confirmed that Iran is enriching uranium at an undergound site near the village of Fordow, in the most serious breach yet of the 2015 nuclear deal.
Iran announced last week that it was doing so, and it was confirmed in an IAEA report released on Monday.
The Fordow site is heavily fortified in a mountain and is believed to be impenetrable by most conventional weapons.
This is the latest in a series of steps Tehran has taken since July to move away from the nuclear deal, which the U.S. withdrew from last year.