US National Security Adviser Mike Waltz has taken responsibility for a group chat in which high-ranking officials planned military strikes in Yemen in the company of a journalist who was inadvertently added.

“I take full responsibility. I built the group,” Waltz told Fox News on Tuesday, adding it was “embarrassing”.

President Donald Trump and US intelligence chiefs have downplayed the security risks and said no classified material was shared.

But Democrats and some Republicans have called for an investigation into what several lawmakers have described as a major breach.

Atlantic magazine editor-in-chief Jeffrey Goldberg reported that he was accidentally added to the Signal chat by a user named Mike Waltz.

In his article that broke the story he says he saw classified military plans for US strikes in Yemen, including weapons packages, targets and timing, two hours before the bombs struck. That content was held back from the piece.

Waltz was unable to explain in his Fox News interview how Goldberg came to be on the chat but – contradicting Trump – he said a member of his staff was not responsible and another, unnamed contact of his was supposed to be there in Goldberg’s place.

“We’ve got the best technical minds looking at how this happened,” Waltz continued, adding that Goldberg’s number had not been on his phone.

“I can tell you for 100% I don’t know this guy,” Waltz said, adding that he had spoken to Elon Musk for help in finding out what happened.

President Trump played down the incident, calling it a “glitch” that had “no impact at all” operationally.

Speaking to Newsmax, Trump said somebody who worked with Mike Waltz at a lower level had Goldberg’s phone number.

US Director of National Intelligence Tulsi Gabbard and CIA Director John Ratcliffe, who were part of the group, denied at a Senate hearing on Tuesday that any classified information was shared in the message chain.

The Signal group chat also included Vice-President JD Vance and Defence Secretary Pete Hegseth.

Mark Warner, Democratic vice-chairman of the Senate Intelligence Committee, said: “This Signal chat situation sheds light on a sloppy and grossly incompetent national security strategy from the Trump administration.”

In his reporting, Goldberg said the officials on the chat had discussed the potential for Europe to pay for US protection of key shipping lanes.

“Whether it’s now or several weeks from now, it will have to be the United States that reopens these shipping lanes,” the account associated with Waltz wrote on 14 March.

He added his team was working with the defence and state departments “to determine how to compile the cost associated and levy them on the Europeans” – at Trump’s request.

At one point in the thread the Vance account griped that the strikes would benefit the Europeans, because of their reliance on those shipping lanes, adding: “I just hate bailing Europe out again.”

The user identified as Hegseth responded three minutes later: “VP: I fully share your loathing of European free-loading. It’s PATHETIC.”

The revelation has sent shockwaves through Washington, prompting a lawsuit and questions about why high-ranking officials discussed such sensitive matters on a potentially vulnerable civilian app.

Some national security experts have argued that the leak was a major operational lapse, and archive experts warned that it violated laws on presidential record keeping.

American Oversight, non-partisan watchdog group, sued the officials who participated in the chat for alleged violations of the Federal Records Act and Administrative Procedure Act.

The group said that by setting the chat to automatically delete messages, the group violated a law requiring White House officials to submit their records to the National Archives.

The National Security Agency warned employees only last month of vulnerabilities in Signal, according to documents obtained by the BBC’s US partner CBS.

With additional reporting by Kayla Epstein, Bernd Debusmann Jr and Brandon Drenon