The U.S. official, who spoke on condition of anonymity, was commenting on a report by South Korea's Yonhap news agency that all three of the North's top military officials were believed to have been replaced.
Trump on Friday revived the summit after canceling it a week earlier. The United States is seeking a negotiated end to North Korea's nuclear weapons program.
U.S. officials believe there was some dissension in the military about Kim's approaches to South Korea and the United States.
The U.S. official did not identify the three military officials. Yonhap identified them as defense chief Pak Yong Sik; Ri Myong Su, chief of the Korean People's Army's (KPA) general staff; and Kim Jong Gak, director of the KPA's General Political Bureau.
Trump wants North Korea to "denuclearize," meaning to get rid of its nuclear arsenal, in return for relief from economic sanctions. North Korea's leadership is believed to regard nuclear weapons as crucial to its survival.
Citing an unnamed intelligence official, Yonhap said No Kwang Chol, first vice minister of the Ministry of People's Armed Forces, had replaced Pak Yong Sik as defense chief, while Ri Myong Su was replaced by his deputy, Ri Yong Gil.
The White House, State Department, CIA and Office of the Director of National Intelligence did not immediately respond to requests for official comment.
South Korea's unification and defense ministries declined to confirm the report, while an official at the Unification Ministry said the government was watching the leadership situation in the North very closely.
All of the newly promoted officials are younger than their predecessors, according to Yonhap, especially Ri Yong Gil, 63, who is 21 years younger than Ri Myong Su.
More at https://www.cnbc.com/2018/06/03/north-koreas-top-three-military-officials-replaced-us-official-says.html