EU spokesman Peter Stano said Brussels noted the steps taken by Iran "with deep concern" and that they represent "a significant departure from Iran's nuclear commitments" under the pact.

But he also insisted that "the strict verification and transparency measures remain in place" under the deal.

"We will redouble our efforts to preserve the agreement and return to its full implementation by all parties," Stano said.

The UN's International Atomic Energy Agency (IAEA) confirmed Monday that Iran had begun further enriching uranium at its Fordo plant after Tehran announced the move.

The step was the latest and most important step the country has taken away from the deal since President Donald Trump withdrew the US from the accord in 2018 and imposed harsh sanctions.

Other parties to the 2015 agreement including the EU, UK, France and Germany have been scrambling to keep alive the accord aimed at curbing Iran's nuclear programme.