EU spokesman Peter Stano said Brussels would wait until a briefing from the director of the UN's IAEA nuclear watchdog later in the day before deciding what action to take.

Earlier, an Iranian government spokesman said the Shahid Alimohammadi enrichment complex in Fordow had begun the "process for producing" uranium enriched to 20 percent.

That would be well above the 3.67 percent cap set in the deal, known as the JCPOA, or Joint Comprehensive Plan of Action.

"If this announcement is going to be implemented... it would constitute a considerable departure from Iran's nuclear commitments under the JCPOA," Stano told reporters.

This would have "serious nuclear non-proliferation implications".

Under the JCPOA, Iran agreed to curtail its nuclear program to levels well under the capacity to build atomic bombs in return for sanctions relief from the international parties to the accord.

The United States has dropped out of the agreement, but the EU and the other signatories -- France, Britain, Germany, Russia and China -- have kept it alive.

A spokesman for the IAEA in Vienna told AFP the agency had been monitoring activity at Fordow and that Director General Rafael Grossi would submit a report to IAEA member states later on Monday.

Stano said Brussels would await that briefing before deciding what action to take.

"The IAEA is the recognised independent monitoring and verification authority for Iran's nuclear programme. And on its assessments, we are basing our decisions and our actions," he said.