President Donald Trump said on Saturday that Interior Secretary Ryan Zinke will step down at the end of December, the latest senior official to exit Trump's high-turnover administration.

An ex-congressman from Montana, Zinke has faced scrutiny over several matters. Trump gave no reason for Zinke's departure after announcing it on Twitter. Trump said he would name a new Interior Secretary next week.

On Friday, Trump said Mick Mulvaney, head of the Office of Management and Budget (OMB), would become acting White House chief of staff.

Trump's White House has had the highest turnover of senior-level staff of the past five presidents, according to figures compiled by the Brookings Institution think tank.

Here are some senior figures who have been fired, quit or otherwise changed roles in the administration:

2018

Ryan Zinke - He will leave at the end of 2018 after being Interior Secretary since early 2017. In the job, he moved to encourage oil drilling and coal mining despite environmental protests. He also faced investigations into his use of security details, chartered flights and a real estate deal.

Mick Mulvaney - After several top candidates declined to become White House chief of staff, Trump said on Friday that Mulvaney would fill the role on an acting basis. A former congressman from South Carolina, Mulvaney has been heading up Trump's OMB and will retain that post, but delegate day-to-day duties there to a deputy. Mulvaney also filled in for about a year as head of the Consumer Financial Protection Bureau (CFPB).

John Kelly - A retired Marine Corps general, Kelly was hired as White House chief of staff to bring order to the chaotic Trump White House, but ultimately fell out with his boss. Trump announced on Dec. 8 that Kelly would leave at year-end.

Matthew Whitaker - Appointed acting attorney general in November, Whitaker is already on track to leave. Trump said on Dec. 7 he will nominate William Barr to be permanent attorney general, subject to Senate confirmation. Whitaker had replaced Jeff Sessions in the top Justice Department post. As acting attorney general, Whitaker took over supervision of a probe led by Special Counsel Robert Mueller into Russian meddling in the 2016 presidential election and any collusion between Moscow and the Trump campaign. The probe has clouded Trump's presidency.

Jeff Sessions - After months of being attacked and ridiculed by the president for recusing himself from the Mueller prove, the former Alabama Republican senator was finally forced out as attorney general on Nov. 7.

Nikki Haley - She said on Oct. 9 that she would step down at the end of the year as U.S. ambassador to the United Nations. On Dec. 7, Trump put forward State Department spokeswoman Heather Nauert as her successor, subject to Senate review.

Don McGahn - Trump said in August the White House counsel would leave amid strains between the two over the Mueller probe.

Scott Pruitt - The Environmental Protection Agency chief quit on July 5 under fire over a series of ethics controversies.

David Shulkin - White House officials said on March 28 that the Veterans Affairs secretary would resign.

H.R. McMaster - The national security adviser was replaced on March 22 by John Bolton.

Andrew McCabe - The deputy FBI director was fired in mid-March by Sessions.

Rex Tillerson - The secretary of state was fired by Trump on March 13 after long-standing tension between them.

Gary Cohn - The National Economic Council director and former Goldman Sachs president said on March 5 he would resign.

Hope Hicks - The White House communications director, a long-serving and trusted Trump aide, resigned on Feb. 28.

Rob Porter - The White House staff secretary resigned in February after accusations of domestic abuse from former wives.

2017

Omarosa Manigault Newman - The former reality TV star was fired as assistant to the president in December.

Richard Cordray - The CFPB director quit in November.

Tom Price - The Health and Human Services secretary quit under pressure from Trump on Sept. 29 over travel practices.

Stephen Bannon - Trump's chief strategist was fired by Trump in mid-August after clashing with White House moderates.

Anthony Scaramucci - The White House communications director was fired by Trump in July after 10 days on the job.

Reince Priebus - Replaced as chief of staff by Kelly, Priebus lost Trump's confidence after setbacks in Congress.

Sean Spicer - Resigned as White House press secretary in July, ending a turbulent tenure.

Walter Shaub - The head of the U.S. Office of Government Ethics, who clashed with Trump, stepped down in July.

Michael Dubke - Resigned as White House communications director in May.

James Comey - The FBI director, who led the Russia probe before Mueller, was fired by Trump in May.

Katie Walsh - The deputy White House chief of staff was transferred out to a Republican activist group in March.

Michael Flynn - Resigned in February as Trump's national security adviser. Flynn later pleaded guilty to lying to the FBI. He is set to be sentenced on Dec. 18.