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

The 2015 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.

The former Spanish foreign minister stressed that there was "no alternative" to the existing accord, which took a decade of hard-fought negotiations to agree.

"The aim of the dispute resolution mechanism is not to reimpose sanctions. The aim of this mechanism is to resolve issues related to the implementation of the agreement," Borrell told reporters at the European Parliament in Strasbourg.

"Let me say this clearly: the dispute resolution mechanism that is being triggered now is not about reimposing sanctions."

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

But British Prime Minister Boris Johnson said on Tuesday that he would be willing to work on a "Trump deal" to replace the existing accord. Borrell refused to comment on the idea when asked.

US President Donald Trump dealt a devastating blow to the Iran deal -- struck by his predecessor Barack Obama -- by unilaterally withdrawing from it in May 2018 and reimposing swingeing sanctions.