UK Foreign Secretary Boris Johnson has challenged the US to show there is a better alternative to the deal with Iran that limits its nuclear programme.
Following talks in Brussels with his Iranian and European counterparts, he said the 2015 accord was a considerable accomplishment that was preventing Iran acquiring nuclear weapons.
Mr Johnson stressed that Iran was fully in compliance with it.
US President Donald Trump wants to amend the deal or withdraw from it.
In October, he refused to recertify for Congress that Iran was complying, accusing it of “not living up to the spirit” of the agreement.
Why does the EU back the deal?
At a news conference after meeting Iranian Foreign Minister Mohammad Javad Zarif on Thursday, representatives of the EU, the UK, France and Germany reiterated their support for the nuclear deal they helped negotiate.
“The deal is working; it is delivering on its main goal, which means keeping the Iranian nuclear programme in check and under close surveillance,” EU foreign policy chief Federica Mogherini said.
“The unity of the international community is essential to preserve a deal that is working, that is making the world safer and that is preventing a potential nuclear arms race in the region. And we expect all parties to continue to fully implement this agreement.”