China’s defense minister, Gen. Li Shangfu, has not been seen in public in more than two weeks, fueling speculation about further upheaval in the military after the abrupt removal of two top commanders in charge of the country’s nuclear force.

General Li’s absence has raised the possibility that he has been placed under an investigation by the Chinese authorities. He had been expected to take part in a meeting last week in Vietnam, but there was no word of his attendance. Asked by reporters on Friday about General Li’s whereabouts, Mao Ning, a spokeswoman for the Chinese Foreign Ministry, said she had no information.

Just six weeks ago, China’s leader, Xi Jinping, replaced the two most senior commanders of the People’s Liberation Army Rocket Force, which oversees China’s nuclear missiles. The abrupt dismissals suggested that Mr. Xi was seeking to reassert his control over the military and purge perceived corruption, disloyalty and dysfunction from its ranks, analysts have said.

Many experts believe that the military commanders may be accused of corruption, though some have said that suspicions of disloyalty toward Mr. Xi within the People’s Liberation Army, or P.L.A., may be involved. In July, China also dismissed the foreign minister, Qin Gang — another official who had risen rapidly under Mr. Xi — without explanation.

General Li “hasn’t been seen in public for some time, and we suspect it’s linked to corruption,” said a U.S. official in Washington who spoke on condition of anonymity. “Some of the P.L.A.’s enduring problems may be too big for Xi to solve, and they have a real impact on the P.L.A.’s ability to achieve what he wants them to do.”

Mr. Xi still appears politically unassailable, with the Communist Party leadership, military top brass and security services packed with his loyalists. Even so, the sudden downfall of such high-ranking officials has exposed the pitfalls in a system so dominated by a single leader and has raised questions about Mr. Xi’s judgment because the officials in question had been promoted by him.

Su Tzu-yun, an expert on the People’s Liberation Army at the Institute for National Defense and Security Research, a think tank in Taipei that is funded by the Taiwanese government, said he was more than 90 percent sure that General Li had been removed from his post.

“For Xi Jinping, this is a loss of face, and in the Chinese military and across China, people will notice, even if they don’t say so openly,” Mr. Su said. “It’s not going to force him from power, but it will erode his prestige as ruler.”

General Li, 65, was promoted to minister of national defense in March, after late last year joining the Central Military Commission, the council led by Mr. Xi through which the party controls the military.

General Li’s last public appearance was in late August, when he spoke at a forum in Beijing attended by officials from African countries. It is not unusual for People’s Liberation Army commanders to be away from the public spotlight, though as the military’s chief diplomat, the defense minister’s absences are more noticeable.

Reuters reported on Friday, citing anonymous sources, that General Li did not attend the scheduled talks last week with Vietnamese officials — an unusual absence that suggested something might be amiss.

For much of his career, General Li was deeply involved in developing and acquiring the People’s Liberation Army’s growing array of rockets, missiles and other advanced weapons. He appeared to have Mr. Xi’s trust as a weapons expert who, like Mr. Xi, was the son of a veteran in Mao Zedong’s revolutionary forces.

An engineer by training, General Li accumulated a sparkling résumé in rocketry, weapons development and the manned space program. He was appointed the inaugural deputy commander of the Strategic Support Force, which Mr. Xi created in late 2015 as part of a sweeping reorganization of the Chinese military. The Strategic Support Force brings together China’s efforts in new realms of military rivalry, like space, cyberoperations and espionage, advanced communications and psychological warfare.

In 2017, General Li was appointed the director of the Chinese military’s Equipment Development Department, and it was his role there that made him a target of U.S. government sanctions in the next year. Citing his role in acquiring Russian fighter planes and surface-to-air missiles, Washington barred General Li from, among other things, obtaining a U.S. visa.

China has rebuffed invitations from the United States for talks between General Li and the defense secretary, Lloyd J. Austin III, saying that the Biden administration should first lift the sanctions.

“As the lyrics of a well-known Chinese song goes, when friends visit, bring out the fine wine. When jackals and wolves visit, bring out the shotgun,” General Li said at an annual security meeting in Singapore this year.

Rahm Emanuel, the United States’ ambassador to Japan, who has been tweaking Mr. Xi on X, the social media platform formerly known as Twitter, said in an interview on Friday that the Chinese government should explain General Li’s absence and also why Mr. Xi skipped the Group of 20 summit last week in India and other commitments.

In a post on X on Friday, Mr. Emanuel asked if General Li had been placed under house arrest.

“There’s just too much,” Mr. Emanuel said. “When you know the history of China, given all the tension economically and internally, people are being arrested left and right.”

Motoko Rich contributed reporting from Tokyo, Sui-Lee Wee from Bangkok and Eric Schmitt contributed from Washington, D.C.