Trump insists there is 'no chaos' at White House

He will bring his military experience to an administration weighed down by a stalled legislative agenda, a cabal of infighting West Wing aides and a stack of investigations.

While Mr Trump is looking for a reset, he pushed back against criticism of his administration on Twitter on Monday.

He said: "Highest Stock Market EVER, best economic numbers in years, unemployment lowest in 17 years, wages raising, border secure, S.C.: No WH chaos!"

Mr Kelly's success in a chaotic White House will depend on how much authority he is granted and whether Mr Trump's duelling aides will put aside their rivalries to work together.

It is also unclear whether a new chief of staff will have any influence over the president's social media histrionics.

Former Trump campaign manager Corey Lewandowski, who was removed from the campaign in June 2016, said on NBC's Meet The Press that he expected Mr Kelly would "restore order to the staff" but also stressed that Mr Trump was unlikely to change his style.

"I say you have to let Trump be Trump. That is what has made him successful over the last 30 years. That is what the American people voted for," Mr Lewandowski said.

Highest Stock Market EVER, best economic numbers in years, unemployment lowest in 17 years, wages raising, border secure, S.C.: No WH chaos!

— Donald J. Trump (@realDonaldTrump) July 31, 2017

"And anybody who thinks they're going to change Donald Trump doesn't know Donald Trump."

Mr Kelly's start follows a tumultuous week, marked by a profane tirade from the new communications director, Mr Trump's continued attacks on his attorney general and the failed effort by Senate Republicans to overhaul the nation's healthcare law.

In addition to strain in the West Wing and with Congress, Mr Kelly starts his new job as tensions escalate with North Korea.

The United States flew two supersonic bombers over the Korean Peninsula on Sunday in a show of force against North Korea, following the country's latest intercontinental ballistic missile test.

The US also said it conducted a successful test of a missile defence system located in Alaska.

Democratic Senator Dianne Feinstein said on CBS's Face The Nation that she hopes Mr Kelly can "be effective", and "begin some very serious negotiation with the North and stop this programme".

Another diplomatic fissure opened on Sunday when Russian President Vladimir Putin said the US would have to cut its embassy and consulate staff in Russia by several hundred under new sanctions from Moscow.

In a television interview, Mr Putin indicated the cutback was retaliation for new sanctions in a bill passed by Congress and sent to Mr Trump.

Mr Trump plans to sign the measure into law, the White House has said.

After Mr Putin's remarks, the State Department deemed the cutbacks "a regrettable and uncalled for act" and said officials would assess the impact and how to respond to it.

While Mr Trump is trying to refresh his team, he signalled that he does not want to give up the fight on healthcare.

On Twitter on Sunday, he said: "Don't give up Republican Senators, the World is watching: Repeal & Replace."

The protracted healthcare fight has slowed Mr Trump's other policy goals, including a tax overhaul and infrastructure investment.

PA