The statement by EU diplomatic chief Federica Mogherini and the foreign ministers of Britain, France and Germany said it would also work to keep "effective financial channels" open with Iran.

"We deeply regret the re-imposition of sanctions by the US, due to the latter's withdrawal from the Joint Comprehensive Plan of Action (JCPOA)," the statement issued in Brussels said.

The EU said it would now swiftly bring in legal cover for firms in the 28-nation bloc to work with Iran, after the Trump administration rejected European calls for an exemption.

"We are determined to protect European economic operators engaged in legitimate business with Iran," the statement added.

"This is why the European Union's updated Blocking Statute enters into force on 7 August to protect EU companies doing legitimate business with Iran from the impact of US extra-territorial sanctions."

The "blocking statute" forbids EU firms from complying with US sanctions, allowing them to recover damages from such penalties and nullifying any foreign court rulings against them.

French carmaker Renault, which does not sell cars in the US, has said it will remain in Iran despite the sanctions. But French oil group Total and carmaker PSA have already indicated they are likely to pull out of Iran.

Despite the US move to restore the sanctions, which Secretary of State Mike Pompeo reaffirmed in Alaska late Sunday, the Europeans said they would continue to work with Iran to preserve the deal.

"The remaining parties to the JCPOA have committed to work on, inter alia, the preservation and maintenance of effective financial channels with Iran, and the continuation of Iran's export of oil and gas," it said.