US President Donald Trump confirmed on Thursday that he was replacing his national security advisor, Mike Waltz, following a chat group leak, saying he planned to move him to the United Nations.

In the first major cabinet shake-up of Trump's new term, the president said Secretary of State Marco Rubio would now also serve as his "interim" national security advisor following Waltz's departure.

"I am pleased to announce that I will be nominating Mike Waltz to be the next United States Ambassador to the United Nations," Trump said on Truth Social, confirming earlier reports that Waltz was being ousted.

"Mike Waltz has worked hard to put our Nation's Interests first. I know he will do the same in his new role."

Trump did not give a reason for the move, but Waltz had been under pressure over the so-called "Signalgate" scandal since late March.

The editor-in-chief of The Atlantic magazine revealed at the time that Waltz had mistakenly added him to a group chat about US strikes on Yemen's Houthi rebels on the commercial messaging app Signal.

Officials in the group laid out the attack plan, including the timings that US warplanes would take off to bomb targets, with the first texts barely half an hour before they launched.

Despite intense media speculation that Trump would fire Waltz over the scandal, the president repeatedly offered his backing, and the national security advisor appeared to have ridden out the storm.

In the end, however, the 51-year-old former congressman from Florida lasted just over 100 days of Trump's second term, which has so far been more stable in terms of personnel than his first.

Democrats will now turn up the heat on Defence Secretary Pete Hegseth, who was the official who revealed the air strike details in advance, and who was also reported to have shared those details in a separate Signal group chat that included, among others, his spouse.

"Now do Hegseth," top Senate Democrat Chuck Schumer posted on X.

Waltz's new role will also require Senate confirmation, ensuring that Signalgate will stay in the headlines.

There was no immediate confirmation of US media reports that Waltz's deputy, Alex Wong, would also leave the National Security Council.

US media had reported that Steve Witkoff, a real estate magnate whom Trump has picked to lead US talks with both Russia and Iran, is in contention to replace Waltz in the longer term. (AFP)