President Trump characterized an extraordinary security breach as a minor transgression on Tuesday, insisting that top administration officials had not shared any classified information as they discussed secret military plans in a group chat that included the editor in chief of The Atlantic magazine.

“So this was not classified,” Mr. Trump said during a meeting with U.S. ambassadors at the White House. “Now if it’s classified information, it’s probably a little bit different, but I always say, you have to learn from every experience.”

Mr. Trump also stood by his national security adviser, Michael Waltz, who had inadvertently added the journalist Jeffrey Goldberg to the chat on the Signal app, which included Vice President JD Vance and others. In the chat, Defense Secretary Pete Hegseth shared information on timing, targets and weapons systems to be used in an attack on Houthi militants in Yemen, according to Mr. Goldberg.

“I think it was very unfair the way they attacked Michael,” the president said of Mr. Waltz.

Former national security officials said they were skeptical that the information shared by Mr. Hegseth ahead of the March 15 strike was not classified, given the life-or-death nature of the operation.

The president and the secretary of defense have the ability to assert, even retroactively, that information is declassified. But officials have refused to answer questions about the specifics of the information or who, exactly, determined that it was unclassified and could be shared on Signal, an encrypted commercial app.

Mr. Hegseth denounced Mr. Goldberg late Monday, saying he had been “peddling hoaxes time and time again.” But on Tuesday morning, testifying in the Senate, the nation’s top two intelligence officials conceded that the exchanges released by The Atlantic were accurate.