But Ri and the two other generals who reportedly were promoted to top positions - No Kwang Chol and Kim Su Gil - have been seen at major party and military events with the North's leader over the past two years, according to 38 North, a website that closely follows North Korean affairs.
The US official did not identify the three military officials.
Citing an intelligence source, Yonhap named the outgoing officials as defense minister Pak Yong Sik, Ri Myong Su, the Korean People's Army's (KPA) chief of the general staff, and Kim Jong Gak, director of the KPA's General Political Bureau and former defense minister.
Assad was quoted as saying: "I am going to visit the Democratic People's Republic of Korea (DPRK) and meet Kim Jong-un".
This could be a sign that the North Korean leader is "pursuing a new policy to become a developing country without nuclear weapons, rather than a poor country with nuclear weapons", he said.
"We can anticipate, at best, a bumpy road to the (negotiations)", Mattis said before the start of a meeting with his South Korean and Japanese counterparts on the sidelines of Shangri-la dialogue in Singapore.
Trump said one thing that could come out of the summit is an agreement formally ending the 1950-53 Korean War, which was concluded only with a truce, not a peace treaty.
There were also widespread reports that North Korea helped Syria build a nuclear plant that was destroyed by Israeli bombing in 2007.
Warriors survive a James scare to win
The Cleveland Cavaliers lost to the Golden State Warriors in heartbreaking fashion in Game 1 of the NBA Finals on Thursday. George Hill was fouled with 4.7 seconds remaining in the game and the Warriors clinging to a one-point lead.
Russian Federation cautions US, North Korea against deliberately pushing unrealistic demands
With respect to US-North Korea summit Mike Pompeo has said there is still a lot of work left to do. North Korea's conciliatory response to that letter appears to have put the summit back on track.
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Speaking to Britain's Harper's Bazaar , she said: "Honestly, sometimes I think I still have to deal with it". But as the match wore on, and rust fell from the American's game, she single-mindedly imposed her will.
The Trump administration wants the North to "denuclearise", 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, while Kim has said he plans to focus on economic development.
Trump met with former North Korean spy chief Kim Yong Chol for about 90 minutes Friday - the highest-level North Korean official to visit the U.S. in 18 years.
U.S. officials believe there was some dissension in the military about Kim's approaches to South Korea and the US.
Pyongyang has long argued that it needs nuclear weapons to protect itself against a possible invasion by the US.
The announcement also reflects Mr Kim's push to shed his reclusive image and seek wider contacts, including apparent efforts underway to hold talks with Russian President Vladimir Putin.
Lower-level U.S. -North Korean talks to prepare for the summit are continuing but have made only "halting progress", according to a second U.S. official briefed on the discussions.
However, US Secretary of Defense, Jim Mattis, affirmed on Sunday at an Asian Security Conference, that sanctions on North Korea would only be relaxed after "verifiable and irreversible steps" toward Pyongyang's denuclearization.
Yonhap reported that Kim Yong Chol had transited Beijing on Monday on his way back to Pyongyang from the United States.