Ryan Zinke will be leaving Donald Trump’s White House administration amid pressure to step down from the Interior Department over allegations of ethics violations.

It was originally believed the US interior secretary would announce the decision on Wednesday, but on Saturday morning the president tweeted: “Secretary of the Interior @RyanZinke will be leaving the Administration at the end of the year after having served for a period of almost two years.

“Ryan has accomplished much during his tenure and I want to thank him for his service to our Nation.”

The move arrived amid vows from Democrats to launch numerous ethics probes, which would have resulted in costly legal fees for Mr Zinke – a factor in his decision to resign from the department, Bloomberg reported.

The president also tweeted that a decision about his replacement would be announced next week.

Mr Zinke, a former Republican congressman from Montana, is leaving his post weeks before Democrats take control of the House, a shift in power that promised to intensify probes into his conduct.

He played a leading part in the president’s efforts to roll back environmental regulations and promote domestic energy development.

His departure comes amid a staff shake-up as Mr Trump heads into his third year in office. The president on Friday named budget director Mick Mulvaney as chief of staff, replacing John Kelly, who will be leaving his post by the new year.

The interior secretary previously declared this year he was “100 per cent confident” no wrongdoing would be found in the pending investigations against him.

Mr Zinke faces a number of probes by federal investigators, including one involving dealings between a foundation he created and the chairman of an energy company.

“I’m 100 percent confident that every investigation will always end up in the same conclusion, which is that I follow all rules, procedures and, most importantly, the law,” Mr Zinke said in a November interview. “I have no desire to leave. I know I’m effective and doing the right thing.”

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Arizona Democratic Congressman Raul Grijalva, who is poised to become chairman of the House Committee on Natural Resources, has said he wants the committee to investigate Mr Zinke as well as the administration’s pro-industry policies, which he said has profited mining, oil, gas and other companies.

“It’s the conflicts that are inherent in that policy – who it favours and the undercutting of basic environmental laws,” Mr Grijalva said. The lawmaker said he intends to expand the committee’s staff to have the capacity to handle the investigations.

Mr Grijalva said he’s particularly troubled by the Montana land deal, which involves a foundation Mr Zinke created and the chairman of Halliburton, a major player in the energy industry that has frequent dealings with Interior Department.

At least one of the pending investigations against the interior secretary has reportedly been referred by the department’s inspector general to the Department of Justice. That move signalled a potential escalation of the investigations ahead of the Democratic takeover in 2019.

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Industry leaders and Republicans in Congress have remained supportive of Mr Zinke, notwithstanding the stream of investigations.

Since taking office, he’s answered their calls to scale back regulations imposed under former president Barack Obama, easing restrictions on greenhouse gas pollution and safety rules for offshore oil and gas drilling.