WASHINGTON — After months of quietly urging the Trump administration to preserve the Iran nuclear deal, European diplomats have begun an aggressive defense of it, warning of serious consequences if the United States ignores their concerns.

President Trump denounced the nuclear agreement in a speech to the United Nations last week as an “embarrassment to the United States.” But he said later that his real goal is not to scrap the deal right away but to enlist the European allies who were also signatories of the 2015 agreement in an effort toughen its provisions.

Responding to his speech, Federica Mogherini, the European Union’s foreign policy chief, said there was “no need to reopen the agreement because it’s fully delivering.” And she expressed the near-disbelief among European officials that Mr. Trump might walk away from the Iran accord while trying to resolve an escalating crisis involving North Korea’s nuclear and missile programs.

“We already have one potential nuclear crisis,” she said. “We definitely do not need to go into a second one.”