BRUSSELS — On his first full day as secretary of state, Mike Pompeo traveled to Europe and delivered a warning that leaders here have come to dread: President Trump will soon pull out of the Iran nuclear deal unless major changes are made.

“Absent a substantial fix, absent overcoming the shortcomings, the flaws of the deal, he is unlikely to stay in that deal past this May,” Mr. Pompeo said of Mr. Trump.

Mr. Pompeo said that Mr. Trump had not made a decision on the accord, but Mr. Pompeo’s itinerary on his first overseas trip as the nation’s chief diplomat — a stop here at NATO headquarters and others in Saudi Arabia, Israel and Jordan in the next three days — suggested that he was preparing allies for a likely decision by Mr. Trump to pull out of the deal.

This week, President Emmanuel Macron of France predicted that Mr. Trump “will get rid of this deal on his own for domestic reasons.” Mr. Trump has set May 12 as his deadline to announce his choice.