A powerful US Senate Republican has called for an independent investigation of Trump administration officials' discussion of sensitive attack plans on the Signal messaging app.

It comes after critics argued that US troops could have died if the information had fallen into the wrong hands.

Senator Roger Wicker of Mississippi, chair of the Senate Armed Services Committee, said he and Senator Jack Reed, the panel's top Democrat, would ask President Donald Trump's administration to expedite an Inspector General report and provide a classified briefing.

"We are signing a letter today asking the administration to expedite an IG report back to the committee. We're sending a similar letter to the administration in an attempt to get ground truth," Mr Wicker told reporters at the Capitol.

"The information as published recently appears to me to be of such a sensitive nature that, based on my knowledge, I would have wanted it classified," he said.

The Defense Department's inspector general, a nonpartisan official charged with rooting out waste, fraud and abuse, was one of several officials Mr Trump has fired since he began his second term in January. He has not been replaced.

Mr Wicker said he was nonetheless confident that the Pentagon would go through with an inspector general report.

A few of Mr Trump's fellow Republicans have joined Democrats in expressing concern about the chat on Signal, an encrypted commercial messaging app, about the planned killing of a Houthi militant in Yemen on 15 March.

The chat included officials such as National Security Adviser Mike Waltz, Vice President JD Vance, Director of National Intelligence Tulsi Gabbard, CIA Director John Ratcliffe and Defense Secretary Pete Hegseth, who did not know that Jeffrey Goldberg, editor-in-chief of the Atlantic, was inadvertently included.

Rubio says journalist inclusion in chat a 'big mistake'

US Secretary of State Marco Rubio has said that the inclusion of the journalist in the chat was a "big mistake" and expected reforms, while highlighting his own limited role.

"Obviously, someone made a mistake - someone made a big mistake - and added a journalist. Nothing against journalists, but you ain't supposed to be on that thing," Mr Rubio told a news conference in Jamaica.

"I think there will be reforms and changes made so this never - this is not going to happen again," he said.

Mr Rubio did not assign blame, but quickly noted that he only participated twice in the chat - once to assign a representative and later after the US government had publicly announced the strikes on Yemen.

Mr Goldberg said he was inadvertently added into the chat on commercial app Signal by Mike Waltz, the national security advisor, and that Defense Secretary Pete Hegseth disclosed strike plans through it.

Earlier, US Director of National Intelligence Tulsi Gabbard described the inclusion of Mr Goldberg to the discussion as a mistake.

Ms Gabbard also told a US House of Representatives' Intelligence Committee hearing on worldwide threats, which was scheduled before news of the controversy broke, that she would be somewhat constrained in her ability to discuss what had happened because a legal case has been filed.

At the hearing, Democrats sought to hold Trump administration officials accountable for the incident, arguing that lives could have been lost if the information had fallen into the wrong hands.

"I think that it's by the awesome grace of God that we are not mourning dead pilots right now," Representative Jim Himes said.

"Everyone here knows that the Russians and the Chinese could have gotten all of that information," he added.

Ms Gabbard and CIA Director John Ratcliffe - who both participated in the chat - are before the committee hearing.

It is the second straight day of testimony from the officials, after a heated session in the Senate where Democrats - and some of President Donald Trump's fellow Republicans - called for accountability and the officials insisted the chat had not included classified information.

Screenshots released by The Atlantic magazine showed that Defense Secretary Pete Hegseth texted the start time for the planned killing of a Houthi militant in Yemen on 15 March along with details of further US airstrikes that would normally be closely guarded secrets.

"This is classified information," Democratic Representative Raja Krishnamoorthi said, calling for Mr Hegseth's resignation.

"It's a weapons system as well as sequence of strikes, as well as details about the operations."

Pressed on the classification issue, Ms Gabbard said it was a matter for the Department of Defense.

"I would point to what was shared would fall under the DoD classification system and the Secretary of Defense's authority," she told the hearing.

Separately, Senate Minority Leader Chuck Schumer and other Democrats in the chamber wrote to President Trump and his top officials urging a Department of Justice probe into how Mr Goldberg was included in the discussion.

"We write to you with extreme alarm about the astonishingly poor judgment shown by your Cabinet and national security advisors," the letter states.

US government officials have said that no classified material was shared in the group chat while Mr Trump voiced support for his national security team.

He also said that his administration would look into the use of the Signal app.

Mr Trump's national security adviser, Mike Waltz, who organised the chat, said that he took "full responsibility" for the breach, but insisted that no classified information was shared.

"I take full responsibility. I built the group; my job is to make sure everything's coordinated," he told Fox News.

Mr Waltz added that he does not personally know Mr Goldberg.

He suggested the leak was the result of him mistakenly saving the journalist's number under another name.

"Have you ever had somebody's contact that shows their name and then you have somebody else's number?" he said.

Meanwhile, Mr Trump said in an interview with Newsmax TV that someone who "worked for Mike Waltz at a lower level" may have had Mr Goldberg's number and was somehow responsible for him ending up in the chat.

Read more:
Magazine publishes entire US attack plan shared in group
Trump team scrambles to handle Signal chat leak fallout
What is Signal and is it secure?

Mr Goldberg said that Mr Hegseth sent information in the Signal chat about targets, weapons and timing ahead of the strikes on 15 March.

The journalist also revealed highly critical comments by top US officials about European allies.

In the chat, a user identified as JD Vance, the US vice president, opposed the strikes saying: "I just hate bailing Europe out again" as countries there were more affected by Houthi attacks on shipping than the United States.

A user identified as Mr Hegseth replies: "I fully share your loathing of European freeloading. It's pathetic".

President Trump said he agreed with Mr Hegseth's comments.

"Yeah I think they've been freeloading," he told reporters. "The European Union's been absolutely terrible to us on trade."

The Houthi rebels, who have controlled much of Yemen for more than a decade, are part of the "axis of resistance" of pro-Iran groups staunchly opposed to Israel and the US.

They have launched scores of drone and missile attacks at ships passing Yemen in the Red Sea and Gulf of Aden during the Gaza war, saying they were carried out in solidarity with Palestinians.