Mr. Trump also stood by his national security adviser, Michael Waltz, who had inadvertently added the journalist Jeffrey Goldberg to the Signal thread, 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.

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

During the meeting, as reporters peppered the president with questions about the leak, Mr. Trump repeatedly turned to Mr. Waltz to answer. Mr. Waltz tried to largely redirect the focus, lauding the strikes in Yemen and attacking Mr. Goldberg.

Michael Waltz, the national security adviser, was the one who inadvertently added a journalist to the Signal chat. “I think it was very unfair the way they attacked Michael,” the president said.Credit…Doug Mills/The New York Times

“This one in particular, I’ve never met, don’t know, never communicated with,” he said, adding that “we are looking into and reviewing how the heck he got into this room.”

Mr. Trump called Mr. Goldberg a “sleazebag.”

The response from Mr. Trump and his allies — downplaying the incident while shifting blame on an opponent — is a timeworn practice they have deployed throughout the president’s political career as they seek to avoid criticism. Mr. Trump is loath to admit mistakes, and he seems wary of firing staff and potentially puncturing his argument that the early months of his second term have been nothing but unalloyed success.

“They’ve made a big deal out of this because we’ve had two perfect months,” Mr. Trump said.

The Trump administration’s account directly contradicted the one given in Mr. Goldberg’s explosive report in The Atlantic on Monday. In his article, Mr. Goldberg shared some screenshots and quotes from the group chat, but said he chose not to share information that could be used to harm American military and intelligence personnel.

On Tuesday, Mr. Goldberg rejected the Trump administration’s assertions that no classified information was shared, saying simply: “They are wrong.”

The administration’s attempt to downplay the gravity of the breach came as Democrats denounced the nation’s top spy chiefs — John Ratcliffe, the director of the C.I.A., and Tulsi Gabbard, the director of national intelligence — during a contentious Senate Intelligence Committee hearing.

National security officials including Tulsi Gabbard, the director of national intelligence, and John Ratcliffe, the C.I.A. director, participated in a contentious Senate Intelligence Committee hearing on Tuesday.Credit…Kenny Holston/The New York Times

Both of them were included on the Signal chat in question.

“This sloppiness, this disrespect for our intelligence agencies is entirely unacceptable,” Senator Michael Bennet of Colorado said on Tuesday during the hearing, which had been scheduled before the report in The Atlantic was published. “You need to do better.”

Senator Mark Warner of Virginia, the committee’s vice chair, said the intelligence officials had displayed “sloppy, careless, incompetent behavior,” while Senator Ron Wyden of Oregon suggested Mr. Hegseth and Mr. Waltz should resign. Many Democrats have resurfaced clips of Mr. Waltz and other Trump allies criticizing Hillary Clinton’s use of a private email server when she was secretary of state.

During the hearing, Mr. Ratcliffe and Ms. Gabbard acknowledged the sensitivity of information about strike targets, but said no sensitive information from their areas of responsibility had been shared.

Mr. Ratcliffe said it was up to Mr. Hegseth, the defense secretary, to determine what information could be shared in an unclassified chat.

Even if the information was not classified, disclosing it in a nonsecure setting could still violate the 1917 Espionage Act. Under that law, what is known as national defense information does not have to be classified for its exposure to be damaging to the country’s security. The Justice Department rarely prosecutes violations that do not involve classified material, and Mr. Trump has stocked his administration with loyalists who have shown little interest in challenging him or his officials.

Senator Mark Warner, Democrat of Virginia, accused the national security officials of “sloppy, careless, incompetent behavior” during the Intelligence Committee hearing.Credit…Kenny Holston/The New York Times

“The Atlantic story is nothing more than a section of the NatSec establishment community running the same, tired gameplay from years past,” Steven Cheung, the White House communications director, posted on social media.

He added that “at every turn anti-Trump forces have tried to weaponize innocuous actions and turn them into faux outrage that Fake News outlets can use to peddle misinformation. Don’t let enemies of America get away with these lies.”

Other top Trump officials and allies have lashed out at Mr. Goldberg, with Mr. Hegseth calling him a “deceitful and highly discredited so-called journalist” and Karoline Leavitt, the White House press secretary, writing on social media that Mr. Goldberg is “well-known for his sensationalist spin.”

Still, several Republicans have expressed concerns about Mr. Goldberg’s inclusion in the chat and acknowledged that it had been a mistake. Most, however, said they wanted a full briefing before drawing any conclusions. Senator Roger Wicker of Mississippi, the chairman of the Armed Services Committee, said his panel would look into what had happened, but provided no details.

“We are definitely concerned,” Mr. Wicker told reporters, adding that he and his colleagues on the committee were “considering our options.”

Mr. Goldberg’s report also raised concerns about administration officials using Signal, a nonsecure messaging platform, and setting the messages to automatically delete. The Presidential Records Act, enacted in 1978, states that the government “shall reserve and retain complete ownership, possession, and control of presidential records,” which includes materials the president’s staff create or receive in the course of their official duties.

Mr. Ratcliffe said the White House and the Cybersecurity and Infrastructure Security Agency had approved the use of Signal for senior officials, and Ms. Leavitt pushed back against those concerns.

“The White House Counsel’s Office has provided guidance on a number of different platforms for President Trump’s top officials to communicate as safely and efficiently as possible,” she wrote on social media.

Mr. Trump said he would prefer for officials to meet in person, but he defended their use of Signal and deflected the blame placed on Mr. Waltz.

“It’s equipment and technology that’s not perfect,” he said.

Mr. Trump also sought to downplay the leak by noting that Mr. Goldberg had left the chat. Mr. Goldberg wrote that he had exited the Signal thread after realizing it “was almost certainly real.”

“The person that was on, as I understand it, he left very early because he didn’t find it very exciting,” Mr. Trump said.

Adam Goldman, Julian Barnes and Robert Jimison contributed reporting.