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.”
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.
Mr Pruitt 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.
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.
He added: “We need him to help @realDonaldTrump drain the regulatory swamp.”