WASHINGTON (AFP) - The White House official widely touted as President Donald Trump's favourite to succeed his outgoing chief of staff John Kelly is instead leaving the administration at year's end.

Mr Nick Ayers, the 36-year-old chief of staff to Vice-President Mike Pence, tweeted that "I will be departing at the end of the year but will work with the #MAGA team to advance the cause", referring to Mr Trump's campaign.

"Thank you @realDonaldTrump, @VP, and my great colleagues for the honor to serve our Nation at The White House."

Mr Trump announced last Saturday (Dec 8) that Mr Kelly, 68, would leave the administration - the latest key personnel move at a time of mounting pressure from the Russia election-meddling probe that comes amid increased focus on preparing for the 2020 elections.

To the President's irritation, Mr Ayers reportedly would not commit to signing on through 2020.

And according to sources cited by The Washington Post, the youthful but politically savvy senior staff member was "sceptical" over taking the position because of the rocky tenures of Mr Kelly and his predecessor Reince Priebus.

When Mr Kelly was picked in July 2017 to replace Mr Priebus, he inherited a White House plagued by political intrigue and internal disorder, and under a cloud because of the allegations of collusion with Russia.

Other potentials on Mr Trump's shortlist include Office of Management and Budget director Mick Mulvaney and Republican congressman Mark Meadows, a leader of the far-right House Freedom Caucus, according to the Post.

The impending departure leaves Mr Trump reliant on a reduced group of key advisers even as he prepares to deal in the new year with a Democratic-controlled House of Representatives.

The opposition party will have the power to launch investigations, issue subpoenas, and generally make his life more difficult.