US President Donald Trump has removed Mike Waltz from his post as national security adviser, and will nominate him as ambassador to the United Nations.

In a post on social media, Trump thanked Waltz for his work and said he would be temporarily replaced by Secretary of State Marco Rubio, who will continue as America's top diplomat.

Waltz had faced criticism for mistakenly adding a journalist to a chat group where sensitive military plans were discussed - a political embarrassment likely to feature during confirmation hearings for the UN post.

The former Florida congressman is the first senior member of the administration to leave the White House in Trump's second term.

"From his time in uniform on the battlefield, in Congress and, as my National Security Advisor, Mike Waltz has worked hard to put our Nation's Interests first," Trump wrote in a post on Truth Social.

"I know he will do the same in his new role."

Waltz posted a short statement on X, alongside a screenshot of the announcement by the president.

"I'm deeply honoured to continue my service to President Trump and our great nation," he wrote.

According to the BBC's US partner CBS News, Trump decided to nominate Waltz as UN ambassador just hours before the announcement on Thursday.

Multiple sources told the network he was ousted because of the Signal situation and a perception in the White House that he did not properly vet National Security Council staff, among other reasons.

But the sources said Trump respects Waltz so he was given a soft landing and a high-profile new post.

However, the BBC spoke to several US officials - who wished to remain anonymous - and they suggested the Trump administration believed Waltz might struggle to be confirmed by the Senate, allowing the president to get rid of him completely without having to fire him.

Waltz has been under scrutiny since he acknowledged in March mistakenly adding the editor-in-chief of the Atlantic magazine, Jeffrey Goldberg, to a group chat on Signal with top US security officials.

Confidential plans for a military strike on Yemen's Houthis were discussed on the message chain, whose members included Waltz, Rubio and Defence Secretary Pete Hegseth.

There was uncertainty on Thursday over the fate of Waltz's deputy, Alex Wong, a seasoned foreign policy hand from Trump's first term who was also on the Signal chat.

Wong was asked about the leak during an interview with the BBC's Newsnight programme on Wednesday. He said the administration had been "very successful" in taking on Yemen's Houthis and "the president led on that".

In March, lawmakers questioned some of the other Signal chat participants at hearings, including the director of national intelligence and the director of the CIA.

The UN ambassador position remains unfilled. Trump withdrew the nomination of his first pick, New York congresswoman Elise Stefanik, amid Republican concerns about preserving their slender majority in the House of Representatives.

Minnesota Governor and former Democratic vice-presidential candidate Tim Walz made light of the national security adviser's exit on Thursday.

He posted on X: "Mike Waltz has left the chat."

Waltz has continued to use Signal, according to an image captured by a Reuters photographer at a White House cabinet meeting on Wednesday.

The zoomed-in picture showed Waltz checking his phone and a chat ongoing with a contact saved under the name of JD Vance, who is US vice-president.

The message from the Vance contact read in part: "I have confirmation from my counterpart it's turned off. He is going to be here."

In an interview with Fox News aired on Thursday evening, Vance rejected any suggestion that Waltz had been fired, saying he was actually being promoted.

"I like Mike," said Vance. "I think he's a great guy. He's got the trust of both me and the president.

"But we also thought that he'd make a better UN ambassador as we get beyond this stage."

Trump's announcement, meanwhile, of the latest role for Rubio appeared to catch state department officials off guard.

Rubio will now be the first official to serve both as secretary of state and national security adviser since Henry Kissinger half a century ago.

Rubio is also acting head of both the gutted United States Agency for International Development and the National Archives.

Some reports suggest Steve Witkoff, a real estate developer and personal friend of Trump who is currently a US Special Envoy to the Middle East, could ultimately replace Waltz.

Another name being touted as a potential candidate by some in Washington is also one of Trump's special envoys, Ric Grenell, who has a longer diplomatic track record.

Trump went through four national security advisers in his first term. The first, Michael Flynn, served for just three weeks.

Another, John Bolton, later wrote an unflattering book about Trump.

Bolton told the BBC on Thursday that Waltz's removal was reminiscent of the "chaos" from Trump's first term. (BBC, excluding headline)

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