U.S. President Donald Trump on Tuesday stood by his national security adviser, Mike Waltz, after The Atlantic's editor-in-chief was accidentally added to Signal group chat where high-level national security officials were discussing military plans in Yemen.

"Michael Waltz has learned a lesson, and he's a good man," Trump said Tuesday in a phone interview with NBC News.

When asked what he was told about how Goldberg came to be added to the Signal chat, Trump said, "It was one of Michael's people on the phone. A staffer had his number on there."

Trump also said Goldberg's presence in the chat had "no impact at all" on the military operation against Houthi forces in Yemen.

Trump's remarks came one day after Jeffrey Goldberg, the editor-in-chief of The Atlantic, said in an article on Monday that senior U.S. national security officials recently added him to a Signal group chat discussing a military strike on Houthi forces in Yemen. He said he became aware of the airstrike plan about two hours before the U.S. operation.

"I have never seen a breach quite like this," said Goldberg in the article.

When asked by media about this, Defense Secretary Pete Hegseth said Monday that "Nobody was texting war plans. And that's all I have to say about that." The defense secretary also lashed out at Goldberg, calling him "a deceitful and highly discredited so-called 'journalist' who's made a profession of peddling hoaxes time and time again."

The incident has sparked grave concerns and harsh criticism.

"If true, this story represents one of the most egregious failures of operational security and common sense I have ever seen," Jack Reed, the Senate Armed Services Committee's top Democrat, said in a statement.

"Military operations need to be handled with utmost discretion, using approved, secure lines of communication, because American lives are on the line. The carelessness shown by President Trump's Cabinet is stunning and dangerous. I will be seeking answers from the administration immediately," said the Democratic senator.

Senate Minority Leader Chuck Schumer described the incident as "amateur behavior" and called for "a full investigation into how this happened and the damage it created."

"This kind of security breach is how people get killed. How our enemies take advantage. How our national security falls into danger. These people are clearly not up for the job," he posted on X.