Iran began testing a second batch of equipment that could be used to enrich uranium, despite possible UN sanctions. The centrifuges could have been tested at any time during the past few months.

But diplomats suspected work was underway because of a recent collapse in talks with the European Union.

The U.S state department called the move “alarming,” saying this could be a step towards large-scale production of enriched uranium. Tehran has repeatedly said its enrichment programme is for peaceful energy production. The West suspects Iran wants to make a nuclear weapon.