The report piles further pressure on Hegseth, who is already under fire over US strikes on alleged drug-trafficking boats that experts say amount to extrajudicial killings.

"The secretary sent nonpublic DoD information identifying the quantity and strike times of manned US aircraft over hostile territory over an unapproved, unsecure network approximately two to four hours before the execution of those strikes," the inspector general's office said in a report, using an abbreviation for the Department of Defense.

"Using a personal cell phone to conduct official business and send nonpublic DoD information through Signal risks potential compromise of sensitive DoD information, which could cause harm to DoD personnel and mission objectives," it said.

Hegseth declined to be interviewed as part of the inspector general's investigation, which was sparked by the Atlantic magazine's revelation in March that its editor-in-chief Jeffrey Goldberg was inadvertently included in a Signal chat in which officials including Hegseth and then-national security advisor Mike Waltz discussed the strikes.

The magazine initially withheld the details the officials discussed, but later published them after the White House insisted that no classified information was shared and attacked Goldberg as a liar.

The inspector general's report noted that Hegseth is an "original classification authority" and can decide when materials no longer require protection, but said that the secretary "sent sensitive, nonpublic, operational information that he determined did not require classification over the Signal chat."

- Trump backs Hegseth -

The chat included messages in which Hegseth revealed the timing of strikes hours before they happened and information on aircraft and missiles involved, while Waltz sent real-time intelligence on the aftermath of the military action.

Trump rejected calls for Hegseth to be fired and largely pinned the blame on Waltz, whom he ultimately replaced as national security advisor, appointing him as US ambassador to the United Nations instead.

US media then reported in April that Hegseth had created a second Signal chat in which the March Yemen strikes were discussed with people including his wife and brother, but the Pentagon chief likewise weathered that storm and remained in office.

The Huthis began targeting shipping in the Red Sea and Gulf of Aden after the Gaza war began in 2023, claiming solidarity with Palestinians.

Huthi attacks prevented ships from passing through the Suez Canal, a vital route that normally carries about 12 percent of world shipping traffic, forcing many companies into a costly detour around southern Africa.

The United States first began conducting strikes in response under the Biden administration, and US forces launched a renewed air campaign against the Huthis on March 15.

Trump's strikes against the Huthis lasted until early May, when a ceasefire agreement was reached with the help of Omani mediation.

Hegseth has also faced increasing recent scrutiny over a September 2 incident in which US forces launched a follow-up strike on the wreckage of a vessel that had already been hit, reportedly killing two survivors.

Both the White House and Pentagon have sought to distance Hegseth from that decision -- which some US lawmakers have said could be a war crime -- instead pinning the blame on the admiral who directly oversaw the operation.