EU leaders meeting in Sofia have backed a "united" approach to keeping the deal alive after US President Donald Trump pulled out and reimposed sanctions, complaining the accord did nothing to stop Iran's ballistic missile programme or interference in Middle East conflicts.

"Everyone in the European Union shares the view that the agreement is not perfect, but that we should remain in this agreement and conduct further negotiations with Iran on the basis of other issues such as the ballistic missile programme," Merkel said as she arrived for the summit.

French President Emmanuel Macron said the bloc was working to keep the existing agreement alive "so that our businesses can remain" in Iran.

This effort would run alongside work to "pursue negotiations on a vital broader agreement," Macron said.

"The 2015 agreement needs to be completed by a nuclear agreement beyond 2025, an agreement on ballistic activities and (Iran's) regional presence," Macron said.

Tehran has warned it is prepared to resume "industrial-scale" uranium enrichment "without any restrictions" unless Europe can provide solid guarantees that it can maintain the economic benefits it gained from the nuclear agreement despite Washington reimposing sanctions.

EU experts have begun work drawing up measures to shield the deal from US sanctions, focusing on nine key issues including ensuring Iran can sell its oil and gas products and have access to international finance.

But given the global reach of US government sanctions it is not clear how effective these measures can be, or whether the EU will try to leverage them as a bargaining chip with Washington.