It is unclear whether Kim Jong-un is removing hardliners - who dominated military policy past year, when Pyongyang-Washington tensions were at their apex - from the high command ahead of his upcoming summit with Donald Trump, or is simply replacing old figures.
Mr Assad, who has frequently been criticised by Mr Trump for his conduct during the protracted Syrian civil war, was quoted by North Korea's KCNA news agency as saying: "I am going to visit the DPRK and meet (His Excellency) Kim Jong-un", using the initials of the country's official name, the Democratic People's Republic of Korea.
But, if confirmed, the moves raise two contrasting scenarios: part of an ongoing reorganization in military leadership by Kim, or possibly a far-reaching intervention to bring in younger military overseers to replace older ranks possibly at odds with his outreach to the United States and its ally South Korea.
Yonhap identified the three officials as defence chief Pak Yong-sik, chief of the Korean People's Army's (KPA) general staff Ri Myong-su, and director of the KPA's General Political Bureau Kim Jong-gak.
Speaking in Singapore at the start of a meeting with the defense ministers of South Korea and Japan, he said, "we can anticipate at best a bumpy road to the negotiations".
The meeting is set to take place June 12 in Singapore.
Assad reportedly made the remarks as he received the credentials of North Korean Ambassador to Syria Mun Jong-nam.
South Korea's Blue House on Monday quashed suggestions a formal end to the Korean War could be announced at the Trump-Kim summit.
Trump on Friday revived the summit after cancelling it a week earlier.
Nabil Fekir Set To Have Medical At Liverpool Today Ahead Of Transfer
French forward Nabil Fekir reacts during the friendly football match between France and Ireland at the Stade de France in Paris, on May 28.
Washington wants North Korea to quickly give up all its nuclear weapons in a verifiable way in return for sanctions and economic relief. North Korea's leadership is believed to regard nuclear weapons as crucial to its survival.
Assad rarely leaves Syria, while no foreign head of state has visited North Korea, since Kim assumed office in 2011, upon the death of his father.
According to the Yonhap news agency, the new North Korean defence minister is No Kwang-chol, a former first vice-minister, replacing Pak Yong-sik.
Pyongyang has long argued that it needs nuclear weapons to protect itself against a possible invasion by the US.
Its nuclear weapons programme has demonstrated advances in ballistic missile technology that experts believe now threatened the U.S. mainland.
There has also been speculation Kim does not wish to leave North Korea for an extended period of time as this may encourage any remaining opponents to his rule to seize the opportunity to stage a coup.
Should real progress be made, South Korean President Moon Jae-in's plan involves the development of three economic belts that would link his country's industrial heartland with the North and then with China and Russian Federation.
That official said US negotiators' efforts to press for definitions of immediate, comprehensive, verifiable and irreversible denuclearisation by North Korea had run into opposition from the White House.