Britain, Germany, France and a senior EU official met Iran's deputy foreign minister Abbas Araghchi in Brussels and "used this opportunity to reiterate their support to the continued full and effective implementation of the (deal) by all sides", the bloc said in a statement.

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

The Brussels meeting was part of "intensive work" to try to maintain the 2015 nuclear deal -- which lifted punishing sanctions in return for Iran ending its nuclear programme -- even if the US quits the deal, the German source said.

Officials and diplomats expect Trump to withdraw US support for the accord, officially known as the Joint Comprehensive Plan of Action (JCPOA), which he has repeatedly criticised.

"For weeks we have been in close contact with E3 partners in particular -- from the working level to the foreign ministers," the German source said.

E3 refers to Germany, Britain and France -- the European signatories to the deal which have led efforts to persuade Trump to stick with a deal which they say is working effectively to prevent Iran obtaining a nuclear weapon.

"In the next few days too, it will be important to keep in touch with all sides in order to prevent an uncontrolled escalation," the German source said.

EU sources told AFP they have been informed in advance that Trump will likely cancel the deal, but they say the full details have not been shared and it is possible he will keep some of the sanctions suspended.

Trump is due to announce his decision at 2:00 pm Washington time (1800 GMT) and the EU's diplomatic chief Federica Mogherini will give the bloc's reaction some time afterwards.