The European Union vowed today to defend a 2015 nuclear deal between Iran and world powers and urged U.S. lawmakers not to reimpose sanctions after President Donald Trump chose not to certify Tehran's compliance with the accord.

Germany and France led a chorus of warnings to the United States, normally the EU's closest foreign policy ally, that any weakening of the agreement to prevent Iran obtaining nuclear weapons could have serious consequences for peace.

EU foreign policy chief Federica Mogherini, who chaired the final phase of the 2015 negotiations, held closed-door talks on how the 28-nation bloc should proceed and ministers were also set to discuss how to tackle Iran's ballistic missile programme.

But while several EU governments, including the Netherlands and Britain, said Iran's ballistic missiles and Tehran's interventions in Syria and Yemen were a concern, ministers said the immediate focus had to be saving the 2015 deal.

Negotiated after 12 years of talks that EU diplomats helped to initiate and carry through, the accord with Iran is the most significant diplomatic success for the bloc in several decades.

At their meeting in Luxembourg, the EU's foreign ministers are also due to approve a new round of economic sanctions on North Korea after Pyongyang's nuclear test last month. Many governments still hold out hope of repeating the Iran deal with North Korea.