The US Senate's top Democrat announced Tuesday he would hold up all Justice Department political appointees in protest over President Donald Trump's acceptance of a $400 million luxury plane as a gift from Qatar.

The Gulf state's royal family has offered to donate a 747-8 jumbo jet as Trump awaits the delayed delivery of two new presidential planes from Boeing.

But the move raises huge constitutional and ethical questions -- as well as security concerns about using an aircraft donated by a foreign power for use as the ultra-sensitive Air Force One.

"This is not just naked corruption, it is also a grave national security threat," Senate Minority Leader Chuck Schumer said in a floor speech Tuesday. 

Schumer's blanket hold -- the first time he has ever resorted to such a move -- applies to dozens of would-be appointees, and could significantly delay their start dates in the Trump administration.

But very few Trump nominees were benefiting from bipartisan agreements to fast-track them through the Senate in any case, meaning Schumer's announcement will likely only have a limited practical impact. 

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

Schumer called for the Justice Department's Foreign Agents Registration Act (FARA) unit to "disclose all activities by Qatari foreign agents inside the US that could benefit" Trump or his businesses.

He also demanded information from the administration about the jet itself, such as whether it comes with security enhancements or would have to be upgraded by the United States. 

"How much will those modifications cost American taxpayers? Hundreds of billions of dollars? Hundreds of millions of dollars? Billions of dollars?" he asked.

And he called for an explanation of what the jet means for future presidential planes that the government has already agreed to buy from Boeing.

The veteran New York Democrat is also demanding that Attorney General Pam Bondi -- who once served as a lobbyist for the Qatari government -- testify before Congress to explain her approval of the gift.

Although Democrats are out of power and cannot block Trump appointees on their own, several have announced holds on groups or individuals that make the confirmation process more drawn-out and unwieldy.

Senator Chris Murphy has also pledged to force votes to block weapons sales to Qatar.

Trump, on a trip this week to the Middle East including Qatar, has long been unhappy with the current Air Force One jets -- two highly customized Boeing 747-200B series aircraft that entered service in 1990 under president George H.W. Bush.

Under questioning from reporters on Monday, Trump angrily defended the arrangement with Qatar, saying he would be "stupid" not to accept such a gift.