WASHINGTON (NYTIMES, WASHINGTON POST, REUTERS) - US President Donald Trump said in a tweet on Saturday (Dec 15) that Mr Ryan Zinke, the secretary of the Interior Department and a key figure in the president's sweeping plan to reshape the nation's environmental framework, would leave the administration at the end of the year.

"Ryan has accomplished much during his tenure and I want to thank him for his service to our Nation," Trump said on Twitter.

"The Trump administration will be announcing the new secretary of the Interior next week," he added.

Mr Zinke, 57, has run the Interior Department overseeing America's vast public lands since early 2017.

He has pursued Trump's agenda to promote oil drilling and coal mining by expanding federal leasing, cutting royalty rates, and easing land protections. He was among Trump's most active Cabinet members, cutting huge wilderness national monuments in Utah to a fraction of their size and proposing offshore oil drilling in the Arctic, Pacific and Atlantic.

He became a darling of the US energy and mining industries and a prime target for conservationists and environmental groups.

Mr Zinke's departure comes amid numerous ethics investigations into his business dealings, travel and policy decisions.

Mr Zinke grew up in Whitefish, Montana, a timber and tourism town at the edge of Glacier National Park.

When he took charge of the Interior Department, he promised that his youth in the West would help him balance conservation and extraction on the land in his care.

Much of that land is in the West, and it includes the national parks and monuments that have become the nation's environmental icons.

With his flashy leadership style and aggressive approach to deregulation, Mr Zinke had largely escaped the public thrashing that Mr Trump has directed at other cabinet members. But in recent weeks the president had signalled that the secretary's tenure might be coming to end, possibly as a result of the ethics inquiries.

"I do want to study whatever is being said," Mr Trump said at a news conference on Nov. 7, referring to the investigations into Mr Zinke.

"I think he's doing an excellent job, but we will take a look at that, and we'll probably have an idea on that in about a week."

Mr Zinke was considered a surprise choice for the position. He had reportedly gained favor with Donald Trump Junior, because, like the president's son, he is an avid game hunter.

Mr Zinke is the latest Trump official to exit an administration beset by questions of ethical conflict. And his departure comes as Mr Trump suggests he will further shake up his White House.

In early November, the president fired Attorney General Jeff Sessions, replacing him with a loyalist who will now take charge of the inquiry into Russian interference in the 2016 presidential election.

Mr Zinke, a former Montana congressman and member of the Navy SEALs best known for riding an Irish sport horse through Washington on his first day in office, oversaw mineral extraction and conservation on roughly 500 million acres of public land.

He had become the subject of several federal investigations, one of which his department's top watchdog has referred to the Justice Department, a potential step towards a criminal investigation.

The inquiries include an examination of a real estate deal involving Mr Zinke's family and a development group backed by the Halliburton chairman David J. Lesar. Mr. Zinke stood to benefit from the deal, while Mr Lesar's oil services company stood to benefit from Mr Zinke's decisions on fossil fuel production.

Mr Zinke has repeatedly denied wrongdoing. "I followed every procedure, every policy, every rule, and most importantly I followed the law," he said in an interview in April.