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.”
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.”
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.
Why is Iran’s missile programme controversial?
The US and EU say Iranian ballistic missile tests conducted in the past year have violated UN Security Council resolution 2231, which endorsed the nuclear deal.
The resolution calls upon Iran not to “undertake any activity related to ballistic missiles designed to be capable of delivering nuclear weapons, including launches using such ballistic missile technology”.
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”.
What does Mr Trump want to change?
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.
Iran says the missiles it has tested are not designed to carry nuclear warheads and insists its nuclear programme is entirely peaceful.
The European ministers expressed serious concern about Iran’s ballistic missile programme, as well as its alleged transfer of missiles and assistance to non-state entities in the Middle East. But they said the issue should be kept separate from the nuclear deal.