The 2015 accord "is delivering on its goal which is guaranteeing that Iran doesn't develop nuclear weapons, the European Union is determined to preserve it," Mogherini said.

EU President Donald Tusk said the Iran deal would be on the agenda when the bloc's leaders meet for a summit in Sofia next week.

"Policies of @realDonaldTrump on #IranDeal and trade will meet a united European approach. EU leaders will tackle both issues at the summit in Sofia next week," Tusk wrote on Twitter.

Mogherini made a direct appeal to the Iranian people and their leaders to stick with the accord after US President Donald Trump said Washington was ditching what he called a "defective" agreement.

"Stay true to your commitments as we will stay true to ours and together with the rest of the international community we will preserve this nuclear deal," Mogherini said at a hastily arranged press conference in Rome.

She warned she was "particularly worried" by Trump's announcement of new sanctions on Iran, saying the EU would act to defend its economic interests.

An EU official said the European Commission, the bloc's executive, would meet next week to discuss possible countermeasures to US sanctions.

The official called Trump's decision "the worst that could have been taken".

European powers Britain, France and Germany led a campaign to persuade Trump to stick with the deal negotiated by his predecessor Barack Obama, arguing that it was the most effective way of stopping Iran developing nuclear weapons.

But the massive diplomatic push ended in failure as Trump -- as expected -- announced the US was pulling out and reimposing sanctions that had been relaxed in return for Iran abandoning its nuclear programme.

But Mogherini said the EU would stick with the accord, which was also signed by Russia and China.

"As long as Iran continues to implement its nuclear related commitments, as it is doing so far, the European Union will remain committed to the continued full and effective implementation of the nuclear deal," she said.

"We fully trust the work, competence and autonomy of the International Atomic Energy Agency that has published 10 reports certifying that Iran has fully complied with its commitments."

Senior British, German, French and EU officials held last-ditch talks with Iran's deputy foreign minister Abbas Araghchi in Brussels just hours before Trump's announcement, stressing that Europe was committed to the deal whatever the news from the White House.

A German foreign ministry source said it was important to keep talks going in coming days to avoid "uncontrolled escalation" after Trump's decision.