WASHINGTON — The European Union’s top diplomat issued yet another stern warning on Tuesday that the Iran nuclear agreement that was reached in 2015 could not be reopened for negotiation or changed, as President Trump has vowed to do.

“Renegotiating part of the agreement or the entire agreement is not an option,” said the diplomat, Federica Mogherini, who serves as the bloc’s high representative for foreign affairs and security policy.

Last month, Mr. Trump promised to walk away from the accord unless it was amended to permanently block Tehran from building nuclear weapons or intercontinental missiles. But Mr. Trump did not scrap the deal entirely, as he has the authority to do, instead kicking to Congress the immediate decision about how to address what he and many Republicans view as flaws in the agreement.

Preserving the nuclear accord is a top priority for the European Union, and Ms. Mogherini said that key legislators on Capitol Hill and White House officials had assured her that the United States would not violate the deal.