Defense Secretary Pete Hegseth used Signal more extensively for official Pentagon business than previously disclosed, engaging in at least a dozen separate chats, people familiar with his management practices said.

In one case, he told aides on the encrypted app to inform foreign governments about an unfolding military operation, the people said. He also used the nongovernmental message service to discuss media appearances, foreign travel, his schedule, and other unclassified but sensitive information, two people said.

The former Fox News host set up many of the chats himself, sending texts from an unsecured line in his Pentagon office and from his personal phone, the two said.

Defense Secretary Pete Hegseth used Signal more extensively for official Pentagon business than previously disclosed, engaging in at least a dozen separate chats, people familiar with his management practices said.

In one case, he told aides on the encrypted app to inform foreign governments about an unfolding military operation, the people said. He also used the nongovernmental message service to discuss media appearances, foreign travel, his schedule, and other unclassified but sensitive information, two people said.

The former Fox News host set up many of the chats himself, sending texts from an unsecured line in his Pentagon office and from his personal phone, the two said.

Some of Hegseth’s messages were posted by his military aide, Marine Col. Ricky Buria, who was given access to the secretary’s personal phone, the people said. It was Buria who posted information in March about an imminent U.S. attack on Houthi militants in Yemen into a Signal chat group that included the secretary’s wife, brother, and private lawyer, the people said.

Hegseth’s frequent use of the app in his daily duties and Buria’s role in posting information on his behalf haven’t been previously reported.

The Pentagon and Buria didn’t respond to requests for comment.

Acting Pentagon Inspector General Steven Stebbins announced last month that he was investigating Hegseth’s use of Signal after the disclosure by Atlantic magazine that the secretary had posted information about the Yemen strikes in a chat involving senior administration officials. Similar information posted in the separate chat that included Hegseth’s family members is also part of the inspector general inquiry.

Among the messages posted in some of the other chats by Hegseth were his thoughts on personnel matters, Pentagon programs facing cuts, and details of administration national security debates.

The texts authorizing aides to tell allies about military operations are among the most sensitive messages he sent, two people said.

Instead of using the Pentagon’s vast communications network, Hegseth preferred Signal to run the Defense Department’s day-to-day operations, the people said. Among those he added to chats were members of his security detail, staffers in his personal office and that of the deputy secretary, as well as public-affairs aides.

To read the messages, aides routinely had to step away from their desks to find a location in the Pentagon that received phone service, which is spotty in the building.

Previous administrations have used the nongovernmental messaging apps, but using Signal to share closely-guarded information could put sensitive information at risk of landing in the wrong hands, experts say.

“The use of personal phones and commercial apps introduces unnecessary risk. Signal is considered unclassified by the government for a reason,” said Marc Polymeropoulos, a former senior U.S. intelligence officer. “It’s clear that U.S. government systems are having a hard time keeping up with the required pace of business.”

In some cases, Hegseth’s messages disappeared without being properly recorded, the people familiar said, potentially a violation of laws requiring official records to be preserved.

A 2023 Pentagon directive restricts the use of some nongovernment apps, including Signal, for official business, saying such messaging services shouldn’t be used for sensitive but unclassified information.

“DOD personnel won’t use non-DOD accounts or personal email accounts, messaging systems or other nonpublic DOD information systems, except approved or authorized government contractor systems, to conduct official business,” the memo from the Pentagon chief information officer said.

It isn’t clear whether Signal has been approved for use by Pentagon officials since the memo was issued.

“This memo isn’t definitive to determine the legality of Secretary Hegseth’s use of Signal on a personal device to transmit nonpublic, unclassified DOD information,” said Aram Gavoor, the associate dean for academic affairs and a national security law professor at George Washington University Law School.

President Trump said last week that he planned to shift national security adviser Mike Waltz, who lost favor within the White House in part because of his role in the Signal controversy, out of his post. Waltz, who inadvertently added the editor of the Atlantic to a Signal group about U.S. strikes in Yemen, will be nominated to be ambassador to the United Nations and is likely to face questions during his confirmation hearing about his and Hegseth’s use of the encrypted app.

Hegseth shared some of the most sensitive information on the Signal chat group that included other senior officials, including specific times that F-18s, MQ-9 Reaper drones and Tomahawk cruise missiles would be used in the March 15 attack, according to a transcript released by the Atlantic. The group was established by Waltz.

In an interview Sunday with NBC’s “Meet the Press,” Trump said Hegseth’s job was “totally safe” and that he wasn’t looking for a new defense chief. “Pete’s going to be great,” Trump said. “He’s doing a great job.”

Write to Alexander Ward at alex.ward@wsj.com and Nancy A. Youssef at nancy.youssef@wsj.com