Josep Borrell, the EU's foreign affairs high representative, spoke after Britain, France and Germany formally triggered a dispute mechanism under the 2015 accord, after Iran announced its fifth major step back from compliance.

The deal gave Iran sanctions relief in return for curbs on its nuclear programme.

Borrell, who will oversee the dispute mechanism which could ultimately lead to reimposition of UN sanctions, said the aim of the process was to save the deal, not sink it.

"The dispute resolution mechanism requires intensive efforts in good faith by all. As the coordinator, I expect all JCPOA participants to approach this process in that spirit," he said, using an acronym for the deal's formal title.

"In light of the ongoing dangerous escalations in the Middle East, the preservation of the JCPOA is now more important than ever," he said.

In launching the process, Britain, France and Germany accused Tehran of repeated violations of the deal but insisted they remained committed to it.