NEW YORK - Erik Prince, the former head of the security contractor Blackwater Worldwide and a prominent supporter of former President Donald Trump, violated a United Nations arms embargo on Libya by sending weapons to a militia commander who was attempting to overthrow the internationally backed government, according to U.N. investigators.

A confidential U.N. report obtained by The New York Times and delivered by investigators to the Security Council on Thursday reveals how Mr. Prince deployed a force of foreign mercenaries, armed with attack aircraft, gunboats and cyberwarfare capabilities, to eastern Libya at the height of a major battle in 2019.

As part of the operation, which the report said cost $80 million, the mercenaries also planned to form a hit squad that could track down and kill selected Libyan commanders.

Mr. Prince, a former Navy SEAL and the brother of Betsy DeVos, Mr. Trump’s education secretary, became a symbol of the excesses of privatized American military force when his Blackwater contractors killed 17 Iraqi civilians in 2007.

In the past decade he has relaunched himself as an executive who strikes deals — sometimes for minerals, other times involving military force — in war-addled but resource-rich countries, mostly in Africa.

During the Trump administration, Mr. Prince was a generous donor and a staunch ally of the president, often in league with figures like Steve Bannon and Roger Stone as they sought to undermine Mr. Trump’s critics. And Mr. Prince came under scrutiny from the Trump-Russia inquiry over his meeting with a Russian banker in 2017.

Mr. Prince refused to cooperate with the U.N. inquiry; his lawyer did not respond to questions about the report. Last year the lawyer, Matthew L. Schwartz, told The Times that Mr. Prince “had nothing whatsoever” to do with military operations in Libya.

As part of the operation, which the report said cost $80 million, the mercenaries also planned to form a hit squad that could track down and kill selected Libyan commanders.

Mr. Prince offered to supply weapons, drones and mercenaries to a Libyan militia commander seeking to overthrow the government, according to U.N. investigators.