US President Donald Trump and his intelligence chiefs have played down a security breach that saw a journalist invited into a Signal group chat where he reported seeing national security officials plan airstrikes in Yemen.

US Director of National Intelligence Tulsi Gabbard and CIA Director John Ratcliffe denied at a Senate hearing that any classified information was shared in the message chain. Defence Secretary Pete Hegseth also faced scrutiny for the messages, though he did not testify.

Democrats on the panel rebuked the cabinet members as "incompetent" with national security.

Over at the White House, Trump stood by National Security Adviser Mike Waltz, who was at the centre of the leak.

Waltz came close to apologising by Tuesday evening, telling Fox News: "I take full responsibility. I built the group."

"It's embarrassing. We're going to get to the bottom of it."

Asked if he had identified who on his staff was at fault, he responded, "a staffer wasn't responsible," and repeated that the error was his "full responsibility".

Waltz also said that he had spoken to Elon Musk, who is leading the unofficial Department of Government Efficiency and has touted himself as "tech support" for the federal government.

"We've got the best technical minds looking at how this happened," Waltz continued, adding that Goldberg "wasn't on my phone".

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.

Atlantic magazine editor-in-chief Jeffrey Goldberg was added to the 18-member group, apparently by accident, and reported that he initially thought it was a hoax.

But he said he realised the messages were authentic once the planned raid was carried out in Yemen.

Some 53 people were killed in the 15 March airstrikes, which US officials said targeted Iran-aligned Houthi rebels who have threatened maritime trade and Israel.

The American raids have continued since then, including early on Tuesday morning.

In addition to Ratcliffe and Gabbard, the Signal group chat included Vice-President JD Vance and White House chief of staff Susie Wiles.

The controversy overshadowed Tuesday's hearing before the Senate Intelligence Committee, which was originally meant to focus on drug cartels and people trafficking.

During the at-times combative session, Ratcliffe said he was not aware of any specific operational information on weapons, targets or timings discussed in the chat, as Goldberg had reported.

Asked if he believed the leak was a huge mistake, Ratcliffe said: "No."

Gabbard repeatedly said "no classified information" was divulged and maintained there was a difference between "inadvertent release" and "malicious leaks" of information.

Both pointed to Hegseth as being the authority on whether the information was classified. Goldberg reported that much of the most sensitive information shared in the chat came from the account under Hegseth's name.

"The Secretary of Defense is the original classification authority for DoD in deciding what would be classified information," Ratcliffe said.

Senate Democrats assailed the Gabbard and Ratcliffe.

Colorado's Michael Bennet accused those involved in the chat of sloppiness, incompetence and disrespect for US intelligence agencies.

Georgia's Jon Ossoff described the episode - which Washington has dubbed Signalgate - as an "embarrassment".

"This is utterly unprofessional. There's been no apology," Ossoff said. "There has been no recognition of the gravity of this error."

Republicans on the panel were far more muted in their misgivings.

"We dodged a bullet," said Senator Lindsey Graham of South Carolina.

Mississippi Republican Roger Wicker, who leads the Senate's armed services committee, later told reporters that lawmakers will investigate the Signal chat leak.

Wicker told reporters that he wants the investigation to be bipartisan and for the committee to have full access to the group chat's transcript.

"We need to find out if it's completely factual, and then make recommendations," he told the NewsNation network. "But I expect we'll have the co-operation of the administration."

Republican Jim Risch of Idaho, who leads the Senate Foreign Relations Committee, also said that he expects the matter to be investigated.

"This is a matter that's going to be investigated, obviously, we're going to know a lot more about it as the facts role out," he said, quoted by The Hill newspaper.

Trump and his White House team cast the controversy as a "co-ordinated effort" to distract from the president's accomplishments.

Throughout the day, Trump played down the leak and defended his national security adviser who was reported to have admitted Goldberg to the group chat.

"Michael Waltz has learned a lesson, and he's a good man," Trump told NBC in a morning phone interview. He also said Goldberg's addition to the group was a "glitch" that had "no impact at all" operationally.

The Republican president indicated it was one of Waltz's aides who had invited the journalist to the chat.

"A staffer had his number on there," said Trump, who has long pilloried reporting by Goldberg going back to the 2020 election.

At an event later at the White House, Trump was joined by Waltz.

"There was no classified information, as I understand it," said the president. "They used an app, if you want to call it an app, that a lot of people use, a lot of people in government use, a lot of people in the media use."

In his own brief remarks, Waltz took aim at Goldberg. He said he had never had any contact with the reporter and accused him of wanting to focus on "more hoaxes", rather than Trump administration successes.

Trump later spoke to Newsmax, where he told the conservative network that "somebody that was on the line with permission, somebody that was with Mike Waltz, worked with Mike Waltz at a lower level, had, I guess Goldberg's" phone number.

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.

On Tuesday, the non-partisan watchdog group American Oversight sued the individual 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.

Signal issued a new statement on Tuesday disputing "vulnerabilities" in its messaging platform.

"Signal is open source, so our code is regularly scrutinized in addition to regular formal audits," the statement said, calling the app "the gold standard for private, secure communications".

Mick Mulroy, former Deputy Assistant Secretary of Defence (DASD) for the Middle East and a retired CIA paramilitary officer, told the BBC that holding sensitive discussions on a "unsure commercial application" was "unacceptable".

"And everyone on that chat knew it," he added. "You do not need to be a member of the military or intelligence community to know that this information is exactly what the enemy would want to know."