Iran said Sunday it would breach the cap "in a few hours" and on Monday announced it had passed 4.5 percent enrichment -- well above the 3.7 percent limit.

It first announced its intention to reduce compliance with the deal in May, a year after US President Donald Trump unilaterally withdrew from the pact and reimposed crippling sanctions.

"We strongly urge Iran to stop and reverse all activities that are inconsistent with the commitments made under the JCPOA," EU spokeswoman Maja Kocijancic told reporters.

The JCPOA or Joint Comprehensive Plan of Action gave Iran relief from punishing sanctions in return for setting strict limits on its nuclear activities.

The remaining signatories -- Britain, France, Germany, Russia and China -- say the deal is the best way to stop Iran developing atomic weapons.

"We are extremely concerned by the announcement made over the weekend by Iran regarding the start of the uranium enrichment above the limit of 3.7 percent," Kocijancic said.

The EU will await a report from the UN's atomic energy authority and consult other signatories to the deal before deciding on future steps, Kocijancic said.

Britain, France and Germany have sought to save the deal by creating special mechanism called INSTEX to sidestep US sanctions so Iran can keep trading.

But the mechanism is still not operational six months after its launch and Tehran has grown increasingly frustrated at Europe's failure to act effectively.

Brussels officials see the latest announcements from Iran as a bid to pressure the West into doing more to help.

"The situation is extremely difficult for them domestically -- they are running out of humanitarian supplies," an EU source told AFP.

"They want to preserve the agreement, but they want the benefits they were promised."

The steps Iran has taken so far -- increasing enrichment and breaching a limit on uranium reserves -- can be reversed, the source said.

However, if Iran went above five percent enrichment, the situation would be more serious, officials warn.