WASHINGTON -- White House officials sounded increasingly doubtful Thursday about the future of embattled Environmental Protection Agency chief Scott Pruitt, even as U.S. President Donald Trump appeared to throw him a public lifeline.

Speaking Thursday to reporters aboard Air Force One, Trump used a series of superlatives to describe Pruitt.

"I think he's done a fantastic job," the president said. "I think he's done an incredible job. He's been very courageous. It hasn't been easy, but I think he's done a fantastic job. I think he'll be fine. "

That was contrasted by more tepid remarks earlier from White House deputy press secretary Hogan Gidley.

"They say we all serve at the pleasure of the president," Gidley told reporters. "The president himself said he had confidence (in Pruitt), and so that's where we stand today."

Pruitt has been under fire for days amid numerous ethics questions, including his rental of a bargain-priced Capitol Hill condo with ties to a fossil fuels lobbyist. If Trump were to fire him, he would be the fourth agency head ousted in the Trump administration's first 15 months.

Trump has often lavished praise on Pruitt, a former Oklahoma attorney general who has worked relentlessly to scrap, delay or rewrite Obama-era environmental regulations opposed by the oil, gas and coal industries.

But he also has publicly expressed support for other administration officials who were fired or resigned, right up until sending tweets announcing their departure.

A review of Pruitt's ethical conduct by White House officials is underway, adding to other probes already being conducted by congressional committees and EPA's inspector general into outsized spending on luxury air travel and unusual security precautions.

The ranking Democrat on the House oversight committee, Rep. Elijah Cummings of Maryland, called Thursday for Pruitt to appear on Capitol Hill next week to provide sworn testimony. In a letter to the committee chairman, Trey Gowdy, a South Carolina Republican, Cummings said EPA had failed to fully comply with prior demands for Pruitt's travel records and requested that the documents be subpoenaed.

The New York Times reported Thursday that at least five EPA officials were reassigned or demoted after pushing back against spending requests that included a $100,000-a-month private jet membership, a bulletproof vehicle and $70,000 to replace two desks in the administrator's office suite. None of those purchases were approved, but Pruitt is reported to have gotten an ornate refurbished desk comparable in grandeur to the one in the Oval Office.

CBS News first reported that the head of Pruitt's security detail was demoted last year after the career employee refused the administrator's demand to use the lights and sirens on his EPA vehicle to get him through D.C. traffic faster.

Meanwhile, an EPA ethics official said Wednesday he wasn't provided all of the relevant "factual information" before determining last week that Pruitt's $50-a-night rental was not an ethics violation.

EPA lawyer Kevin Minoli said his finding that Pruitt was paying fair-market value was based on the assumption that Pruitt occupied only one bedroom for $50 a night, as outlined in the lease.

Media reports later disclosed that Pruitt's college-aged daughter occupied a second bedroom in the unit while she interned at the White House last summer. Minoli said he did not consider the value of a second room in his analysis.

Pruitt paid about $1,000 a month, less than a third of what Minoli's review found nearby two-bedroom homes listed for. The Associated Press obtained a copy of Minoli's letter, which was first reported by CNN.

Pruitt had gone on the offensive Wednesday, trying to shore up his position in a series of interviews with Fox News and conservative media outlets during which he continued to suggest he had lived alone.

White House press secretary Sarah Sanders said Wednesday that Trump is not OK with some of the details that have emerged, including news this week of enormous raises awarded to two of Pruitt's close aides. In a combative Fox News interview, Pruitt said he didn't approve the raises and doesn't know who did.

Rep. Elise Stefanik of New York on Thursday became the third House Republican to say Pruitt should go, joining a growing chorus of Democrats and environmentalists. She was speaking to about 200 constituents in her home district.

Amid the ethics cloud, one of Pruitt's closest aides has resigned. Samantha Dravis served as his senior counsel and associate administrator for policy. EPA spokeswoman Liz Bowman said Thursday that Dravis, 34, was leaving to pursue other opportunities.

Dravis previously worked for a fundraising group founded by Pruitt before being hired at EPA. She often accompanied the administrator on his frequent trips across the country and oversees.

The condo rented by Pruitt is co-owned by the wife of J. Steven Hart, chairman and CEO of the powerhouse lobbying firm Williams & Jensen.

On Pruitt's lease, a copy of which was reviewed by AP, Steven Hart's name was originally typed in as "landlord" but was scratched out. The name of his wife, health care lobbyist Vicki Hart, was scribbled in.

Federal disclosure reports show Hart's firm lobbied EPA, including Pruitt himself, extensively over the past year.

The Associated Press reported last week that while living in the Hart condo he met in his EPA office with a lobbyist from Hart's firm and two executives from an energy company seeking to scuttle tighter pollution standards for coal-fired power plants.

Beyond the question of whether Pruitt paid a fair-market value for the rental, Hart's business interests potentially raise other ethics issues that Minoli said he did not consider as part of his earlier review of whether the favourable lease constituted an improper gift to Pruitt from the lobbyist.

Ethics rules covering federal officials say they must remain impartial when making regulatory decisions and can't show favouritism. Pruitt also signed an ethics pledge when joining the Trump administration in which he promised not to accept gifts from lobbyists.

But, ultimately, it's up to the president to determine whether Pruitt goes or stays.

"I'll make that determination," Trump said when asked whether he was bothered by the ethics issues surrounding Pruitt. "But he's a good man, he's done a terrific job. But I'll take a look at it."

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Associated Press reporters Zeke Miller, Catherine Lucey and Jonathan Lemire contributed to this report.