The European Union's diplomatic chief Federica Mogherini on Thursday insisted the Iran nuclear deal "is working", as US President Donald Trump mulls reimposing sanctions on Tehran.

"The deal is working, it is delivering on its main goal which means keeping the Iranian nuclear programme in check and under close surveillance," Mogherini said after talks in Brussels with Iranian Foreign Minister Mohammad Javad Zarif and his British, French and German counterparts.

Senior officials have told AFP they "expect" Trump to extend waivers on sanctions against Iran on Friday to keep the US in line with the 2015 deal, which the president has repeatedly lambasted.

The European Union and Britain, Germany and France -- which all played a key role in the hard-fought accord -- once again closed ranks to back the deal, which curbed Iran's nuclear ambitions in return for the relaxing of punishing sanctions.

Mogherini said it was vital to preserve an agreement that is "making the world safer and that is preventing a potential nuclear arms race in the region".

While hawks in Washington have called for the agreement to be scrapped, British foreign minister Boris Johnson said that so far no-one has come up with a better alternative.

"We greatly value the JCPOA, the nuclear deal with Iran, we think it is a considerable diplomatic accomplishment, it's a way of stopping Iran from acquiring nuclear weapons and Iran is in compliance with this agreement according to the International Atomic Energy Agency," Johnson said.

German Foreign Minister Sigmar Gabriel said Europe wanted to protect the deal "against every possible undermining decision".

"We know that it's absolutely necessary to have the signal that it's possible by diplomatic approaches to prevent the development of nuclear weapons, in a time when other parts of the world are discussing how to get nuclear weapons into force," Gabriel said.

"It would send a very dangerous signal to the rest of the world if the only agreement that prevents the proliferation of nuclear weapons would be negatively affected."