U.S. Interior Secretary Ryan Zinke, who is facing federal investigations into his travel, political activity and potential conflicts of interest, will leave the administration at year's end, President Donald Trump said Saturday.

Trump, in tweeting Zinke's departure, said the former Montana congressman "accomplished much during his tenure" and that a replacement would be announced next week. The cabinet post requires Senate confirmation.

Zinke is leaving weeks before Democrats take control of the House, a shift in power that promised to intensify probes into his conduct. His departure comes amid a staff shake-up as Trump heads into his third year in office. The president on Friday named White House budget director Mick Mulvaney as his next chief of staff.

Zinke, 57, played a leading part in Trump's efforts to roll back environmental regulations and promote domestic energy development. When he recently travelled to survey damage from California's wildfires, Zinke echoed Trump claims that lax forest management was to blame in the devastation.

Dogged by ethics probes

He pushed to develop oil, natural gas and coal beneath public lands in line with the administration's aims. But Zinke has been dogged by ethics probes, including one centred on a Montana land deal involving a foundation he created and the chair of an energy services company that does business with the Interior Department.

Investigators also are reviewing Zinke's decision to block two tribes from opening a casino in Connecticut and his redrawing of boundaries to shrink a Utah protected area.

Zinke has denied wrongdoing.

The Associated Press reported last month that the department's internal watchdog had referred an investigation of Zinke to the Justice Department.

Zinke had denied hunting for new job

Trump told reporters this fall he was evaluating Zinke's future in the administration in light of the allegations.

Asked by reporters last month whether he might fire Zinke, Trump said, "No, I'm going to look into any complaints."

Zinke in November denied he already was hunting for his next job.

Zinke's departure comes amid a shake-up of Trump's administration, which has had a high rate of turnover. (Kevin Lamarque/Reuters)

"I enjoy working for the president," he told a Montana radio station. "Now, If you do your job, he supports you."

"I think I'm probably going to be the commander of space command," Zinke said. "How's that one?"

Zinke had a memorable administration debut when he rode a horse to his first day of work in March 2017.

Zinke rides on horseback with a U.S. Park Police horse mounted unit while reporting for his first day of work in March 2017. (Timi Heilemann/U.S. Department of Interior via Reuters)

Zinke outlasted Environmental Protection Agency chief Scott Pruitt, another enthusiastic advocate of Trump's way of governing who lost favour with Trump amid ethics scandals. Pruitt resigned in July. Trump's first Health and Human Services secretary, Tom Price, also resigned under a cloud of ethical questions.

Senate Minority Leader Chuck Schumer was scathing in response to the news that Zinke was leaving.

"Ryan Zinke was one of the most toxic members of the cabinet in the way he treated our environment, our precious public lands, and the way he treated the govt like it was his personal honey pot," the New York Democrat tweeted Saturday. "The swamp cabinet will be a little less foul without him."

Neither Zinke nor Interior Department spokespeople immediately returned requests for comment on Saturday.

'Worst Interior secretary in history'

Arizona Representative Raul Grijalva, the top Democrat on the House natural resources committee, had warned that after Democrats took control of the House they intended to call Zinke to testify on his ethics issues.

Earlier this month, ZInke unleashed a jarring personal attack on Grijalva, tweeting, "It's hard for him to think straight from the bottom of the bottle."

Under Zinke's watch, the Interior Department moved to auction off more oil leases, ended a moratorium on new sales of federally owned coal, and repealed mandates governing drilling. Zinke's focus on the president's energy agenda was cheered by oil, gas and mining advocates, who credit the administration with seeking to balance conservation with development on public lands. But his tenure was denounced by most conservation groups.

"Zinke will go down as the worst Interior secretary in history," said Kieran Suckling, executive director of the Center for Biological Diversity, in a statement released Saturday. "His slash-and-burn approach was absolutely destructive for public lands and wildlife. Allowing David Bernhardt to continue to call the shots will still be just as ugly. Different people, same appetite for greed and profit."

Bernhardt, the deputy secretary, is in line to lead the Interior Department on an interim basis. He has spent years in Washington, D.C., as a lobbyist for the oil and gas industry and has deep ties to Republican politicians and conservative interest groups.