US President Donald Trump on Saturday announced his chief of staff John Kelly will soon be leaving 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.

Kelly, a retired Marine Corps general, was long rumoured to be on the way out, amid reports that his relationship with the volatile Trump had deteriorated to such an extent they were no longer on speaking terms.

Some Democrats suggested that Trump timed the announcement to distract from troubling legal filings on Friday from the special counsel investigating Russian meddling.

Prosecutors said Trump had directed his lawyer Michael Cohen to make illegal payments to two women to silence allegations of sexual affairs.

The Kelly news reportedly was originally scheduled for Monday before the president made the impromptu announcement, speaking to reporters on the White House lawn.

“John Kelly will be leaving at the end of the year,” Trump said. “I appreciate his service very much.”

He said a replacement – possibly an interim appointment – would be named “over the next day or two.”

Nick Ayers, the 36-year-old chief of staff to Vice President Mike Pence, is widely touted to be Trump’s favourite to succeed Kelly.

For all his youth, Ayers is said to have the political savvy – crucial as Trump plots a path to the 2020 election – that Kelly, a retired Marine Corps general, lacked.

When Kelly, 68, was tabbed in July last year to replace chief of staff Reince 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.

Kelly is credited with bringing some discipline to the Oval Office.

When he took up his post, for example, the president’s son-in-law Jared Kushner was reportedly holding regular informal conversations with Saudi Crown Prince Mohammed bin Salman – Kelly insisted on returning to past practice requiring a member of the National Security Council to take part in all calls with foreign leaders.

But Kelly’s tenure was hardly smooth sailing. His comments about the Civil War and immigration drew liberals’ wrath.

Democratic Congresswoman Maxine Waters tweeted on Friday that Kelly was “one of a long list of failed & incompetent Trump appointees that served in a dysfunctional White House.”

The job of White House chief of staff is one of the most vital and difficult – and can be one of the most thankless – in any administration.

Kelly once suggested that being named to the position was something of a curse, joking that “God punished me, I guess.”

Trump was said not to have appreciated Kelly’s humour.

The Republican president, who once said he wanted Kelly to stay through 2020, had hinted for weeks that he was no longer enamoured with his aide.

“There are certain things I love what he does and certain things I don’t like that he does,” he said in mid-November, adding: “At some point he’s going to want to move on.”

The impending departure leaves 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.