Calling the plane a "flying palace," ABC News, which first reported the story, said the Boeing 747-8 jumbo jet gifted from the Qatari royal family would possibly be the most expensive gift ever received by the American government.

The controversy around the jet -- and Trump's boast that it was coming "free of charge" -- builds on questions the US leader is facing over potential conflicts of interest with his family businesses and use of public office.

In a social media post late Sunday that made no mention of Qatar, Trump went on the offensive to claim the plane was a temporary "gift" that would go to the Defence Department, and would replace an existing four-decade-old model.

Trump, 78, said the process was unfolding as a "transparent transaction" but did not specify whether any party was receiving something in return, and instead blamed Democrats for wanting to cash out on a new Air Force One unnecessarily.

Qatar swiftly sought to downplay the uproar, saying reports describing the jet as a gift "are inaccurate."

- Ethical, legal questions -

"The possible transfer of an aircraft for temporary use as Air Force One is currently under consideration between Qatar's Ministry of Defense and the US Department of Defense," said Ali Al-Ansari, Qatar's media attache to Washington, stressing that no decision had been made.

The US Constitution prohibits government officials from accepting gifts "from any King, Prince or foreign State," in a section known as the emoluments clause.

But Trump would skirt the law by handing the jet to his presidential library after he leaves office.

The plan is to be announced as Trump travels to three Middle East countries this week, including Qatar, reported ABC and The New York Times.

Acceptance of the gift has raised ethical questions from both sides of the political aisle as it appears to flout laws set up to stanch government corruption.

Laura Loomer, a far-right ally of Trump, said accepting Qatar's plane would be a "stain" on the administration.

"We cannot accept a $400 million 'gift' from jihadists in suits," she posted on X. "The Qataris fund the same Iranian proxies in Hamas and Hezbollah who have murdered US Service Members."

The White House and Department of Justice, however, believe the gift is legal and not a bribe because it is not given in exchange for any particular favor or action, sources told ABC.

And it is not unconstitutional, they say, because it will first be passed to the US Air Force before being turned over to the presidential library, thereby never being gifted to an individual.

- 'Wildly illegal' -

The Democratic National Committee said the move was proof of Trump using the White House for personal financial gain.

"While working families brace for higher costs and empty shelves, Trump is still focusing on enriching himself and his billionaire backers," the DNC said in an email to supporters.

Multiple Democratic lawmakers blasted the plan.

Senator Chris Murphy called it "wildly illegal," while Representative Kelly Morrison said such a gift amounts to "corruption in plain sight" and an unethical, unconstitutional "bribe."

The US president has long been unhappy with the Air Force One jets -- two highly customized Boeing 747-200B series aircraft.

Earlier this year Trump said his administration was "looking at alternatives" to Boeing following delays in the delivery of two new models.

The aerospace giant agreed in 2018 to supply two 747-8 aircraft by the end of 2024 for $3.9 billion -- both ready to transport whoever occupied the White House at that time.

But a subcontractor went bankrupt and the coronavirus pandemic disrupted production, forcing Boeing to push back delivery to 2027 and 2028.

Although the plane the Qataris are offering is reportedly over 10 years old, a new Boeing 747-8 costs around $400 million, experts said.

The aircraft must additionally be outfitted with extensive communications and security upgrades before it becomes Air Force One.