U.S. President Donald Trump ousted his national security adviser Mike Waltz on Thursday and named Secretary of State Marco Rubio as his interim replacement in the first major shakeup of Trump`s inner circle since he took office in January, writes Reuters.

Trump, in a social media post, said he would nominate Waltz to be the next U.S. ambassador to the United Nations, adding that "he has worked hard to put our nation`s interests first".

Earlier in the day, multiple sources said Trump had decided to remove Waltz from his national security post. The retired Army Green Beret and former Republican lawmaker from Florida had faced criticism inside the White House, particularly after he was caught up in a March scandal involving a Signal chat among top Trump national security aides.

Rubio will be the first person since Henry Kissinger in the 1970s to hold the positions of secretary of state and national security adviser simultaneously.

"When I have a problem, I call up Marco. He gets it solved", Trump said at a White House event earlier on Thursday.

A person familiar with the matter said Trump wanted to get to the 100-day mark in his term before firing a cabinet-level official. News of the shake-up on Thursday was so abrupt that State Department spokesperson Tammy Bruce learned about it from reporters at a briefing.