European powers have urged the US not to abandon the 2015 agreement with Iran that limits its nuclear programme, saying it is making the world safer.
After meeting his Iranian, French, German and EU counterparts in Brussels, UK Foreign Secretary Boris Johnson insisted the deal was preventing Iran from acquiring nuclear weapons.
He also challenged Washington to come up with a better alternative.
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”.
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”.
The US president declared in October that the agreement was “one of the worst and most one-sided transactions the United States has ever entered into”, and warned that within a few years Iran would be able to “sprint towards a rapid nuclear weapons breakout”.
He accused Iran of committing “multiple violations” and promised to work with Congress to “address the deal’s many serious flaws”.
Mr Trump said they included the deal’s “sunset clauses”, one of which allows for the lifting of restrictions on Iran’s uranium enrichment programme after 2025.
He also wants to give the International Atomic Energy Agency access to Iranian military sites, and for the deal to cover Iran’s ballistic missile programme.
Critics of the deal in Congress have also proposed amending legislation to ensure that US sanctions would “snap back” automatically if Iran carried out certain actions.
On Friday, Mr Trump is set to decide whether to extend relief for Iran from some US economic sanctions.
The sanctions, which were suspended in 2016, had cut Iran’s central bank out of the international financial system and imposed penalties for buying Iranian oil.
US officials told the Associated Press on Wednesday that Mr Trump was expected to extend the sanctions relief for another 120 days. But they said he might also impose new, targeted sanctions on Iranian businesses and people allegedly involved in missile tests, supporting terrorism, and human rights abuses.