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.
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."
Mr Johnson described the deal, which is known formally as the Joint Comprehensive Plan of Action (JCPOA), as a "considerable diplomatic accomplishment".
"I don't think that anybody has produced a better alternative to the JCPOA as a way of preventing the Iranians from going ahead with the acquisition of a military nuclear capability," he said. "It is incumbent on those who oppose the JCPOA to come up with that better solution because we have not seen it so far."
Iran's foreign ministry said on Tuesday that if the US withdrew from the agreement, it was ready to give an "appropriate and heavy response".
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