An EU spokeswoman repeated that it was becoming "increasingly difficult" to save the accord, which was abandoned by the US in May last year and which Iran has undercut with a series of recent moves to step up its nuclear activities.

After the US ditched the deal it reimposed crippling sanctions, prompting Iran to begin suspending its own commitments, with President Hassan Rouhani saying Tuesday it would restart enrichment at the Fordow plant south of Tehran.

"We are concerned by President Rouhani's announcement today to further reduce Iran's commitments under the JCPOA," EU spokeswoman Maja Kocijancic told reporters, using an abbreviation for the deal's official title, the Joint Comprehensive Plan of Action.

"We urge Iran to reverse all activities that are inconsistent with its commitments under the JCPOA and to refrain from any further measures that would undermine the preservation and full implementation of the nuclear deal."

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

But, to Tehran's mounting frustration, European efforts to create a way for foreign firms to keep trading with Iran have so far failed to have any significant impact.

Kocijancic said the EU remained committed to the deal but warned this depended on "full compliance" from Iran.

"High Representative Federica Mogherini said herself recently it is becoming increasingly difficult to preserve the JCPOA," Kocijancic warned.

A key factor for EU countries is the assessment of Iran's latest actions by the UN's International Atomic Energy Agency (IAEA), which oversees Tehran's nuclear activities.