Image copyright Reuters Image caption The White House reportedly views the Pruitt situation as "unsustainable"

Pressure is building on embattled US Environmental Protection Agency (EPA) chief Scott Pruitt, who is embroiled in allegations of ethics violations.

President Donald Trump has expressed tepid support for him, but the White House is said to be losing patience.

Mr Pruitt is already under scrutiny for renting an apartment with ties to a fossil fuels lobbyist.

He is also accused of bypassing the White House to secure big pay rises for two long-time staff members.

Asked if he still has confidence in Mr Pruitt while boarding Air Force One on Thursday, President Trump said: "I do."

But deputy White House press secretary Hogan Gidley told Fox News earlier in the day: "I can't speak to the future of Scott Pruitt."

Image copyright Getty Images Image caption Mr Pruitt was paying $50-a-night for a Capitol Hill apartment leased from the wife of an energy lobbyist

The EPA's top ethics official said on Wednesday he was not provided with the full facts when he ruled there was no ethics violation in Mr Pruitt's $50-a-night lease of a Capitol Hill apartment from the wife of an energy lobbyist.

A Democratic congresswoman, who lives in the same Washington condo complex where Mr Pruitt has been staying, recalled an occasion when his security detail broke down a door when Mr Pruitt did not respond to them because he was napping.

A marked man

Anthony Zurcher, BBC News, Washington

In the latest episode of the running soap opera that is the Trump White House, a Washington-based EPA newsletter is reporting that Rob Porter, the exiled former White House staff secretary, is behind the recent spate of revelations damaging to the administration environment chief, Scott Pruitt.

It gets juicier.

According to InsideEPA, Porter's motivation for the leaks is because a former girlfriend, who until last week served as a senior Pruitt adviser, was the one who first informed White House officials of Porter's alleged history of spousal abuse.

The outlook for Mr Pruitt is darkening by the hour. While some conservatives, who approve of his efforts to curtail EPA regulation and trim its budget, are sticking by the former Oklahoma attorney general, support from the White House - publicly and on background - is tepid at best.

Perhaps the strongest card remaining in Mr Pruitt's deck is that there are already three senior vacancies the Trump administration needs to fill - secretary of state, director of the CIA and secretary of veterans affairs.

Adding one more bruising Senate confirmation battle to the docket, when politicians are trying to turn their focus on the upcoming mid-term elections, would be a most unwelcome prospect.

"I know that Congress appropriates money for the EPA to protect human health and the environment - not for repairs to the administrator's residence," Minnesota Democrat Betty McCollum wrote in a letter to the EPA this week, querying how the damage to the door was paid for.

That housing agreement is now being investigated by the department's inspector general.

Mr Pruitt also found himself engaged in a combative exchange with a reporter on Fox News on Wednesday.

He denied orchestrating pay rises - of almost $57,000 and $28,000 - for top aides who have worked with him since he was the attorney general of Oklahoma.

The Trump administration had previously rejected a request to increase the employees' salary, according to the Atlantic.

Mr Pruitt said he only found out about the salary hikes on Tuesday and did not know who made the decision.

But Fox reporter Ed Henry asked: "You don't know? You run the agency. You don't know who did this?"

"I found out about this yesterday, and I corrected the action," Mr Pruitt said.

The Associated Press reports that White House officials regard the Pruitt situation as "unsustainable".

They warned Mr Pruitt before the Fox interview "that if he failed to clear the ethical cloud his job would be in serious doubt", reports the news agency.

Mr Pruitt has also been under scrutiny for his first-class travel, including a $120,000 work trip to Italy last year.

According to CBS News, the head of his security detail was removed after he objected to Mr Pruitt's suggestion shortly after taking office that his motorcade use sirens to get through dense traffic in Washington.

The 16-year security veteran of the EPA was replaced with an agent who argued that Mr Pruitt must fly first-class due to "specific, ongoing threats associated with the Administrator's air travel".

The New York Times on Thursday reported at least five EPA officials were re-assigned, demoted or requested new jobs after raising concerns about Mr Pruitt's spending and management of the agency.

But conservatives such as Senators Rand Paul and Ted Cruz and radio host Rush Limbaugh have rallied to Mr Pruitt's defence.

Mr Trump will not fire Mr Pruitt, because he "is too cagey to be duped and bullied by the Obama groupies", Senator Cruz predicted.

Senator Paul called Mr Pruitt "the bravest and most conservative member of Trump's Cabinet," in a Twitter post.