"Iran's latest actions clearly contravene the deal and pose a risk to our national security," Foreign Secretary Dominic Raab said in a statement.

"We want to find a way forward through constructive international dialogue but Iran needs to stand by the commitments it made and urgently return to full compliance."

Iranian President Hassan Rouhani said earlier Tuesday that the country would restart enrichment at its Fordow plant south of Tehran.

It is the latest move by Tehran to undercut the 2015 international accord limiting its nuclear activities, which was abandoned by the United States in May last year.

After US President Donald Trump ditched the deal, he reimposed crippling sanctions, prompting Iran to begin suspending its own commitments.

The three European parties to the agreement -- France, Britain and Germany -- along with the EU diplomatic service have tried to keep the deal alive despite the sanctions.

The European Union voiced its own concern at the Iranian announcement, warning it is getting harder to preserve the 2015 deal.

France also urged Iran to reverse its decision, noting it was "deeply worried" by its latest steps and that it would await reports from the UN nuclear agency on the situation.