North Korea removed the second most powerful military official, Pak Jong Chon, Reuters reported citing KCNA news agency.
The reasons for the change have not been given by the officials. Pyongyang regularly revamps its leadership and the year-end party gathering has often been used to announce personnel reshuffles and major policy decisions.
Pak, vice chairman of the Central Military Commission of the ruling Workers' Party and a secretary of the party's Central Committee, was replaced by Ri Yong Gil at the committee's annual meeting last week, according to Reuters citing official KCNA news agency.
State television showed Pak sitting in the front row of the podium with his head down during the meeting while other members raised their hands to vote on personnel issues. His seat was later shown unoccupied.
He was also absent in photos released on Monday by the official KCNA news agency of Kim's New Year's Day visit to the Kumsusan Palace of the Sun which houses the bodies of his grandfather and father, according to Reuters.
Pak's replacement came as Kim Jong-un called for the development of new intercontinental ballistic missiles and a larger nuclear arsenal to counter the United States and South Korea as key to the isolated country's 2023 defence strategy.
Kim Jong Un made these remarks during a plenary meeting of the Workers' Party of Korea (WPK).
"The importance and necessity of a mass-producing of tactical nuclear weapons and calls for an exponential increase of the country's nuclear arsenal," KCNA said, as quoted by the South Korean Yonhap news agency on Sunday.
The North Korean leader also called for the development of a new intercontinental ballistic missile (ICBM) in 2023. In addition, Kim Jong Un set out the goal of putting a reconnaissance satellite into orbit as early as possible, according to KCNA.
On Sunday, a short-range ballistic missile (SRBM) test-fired by North Korea flew 400 km, Yonhap reported citing the country's military. The report said the missile was launched from the Ryongsong area of Pyongyang at 2.50 am (local time).
The missile, the first to be test-fired by the Kim Jong-un regime in 2023, was fired into the Sea of Japan and flew some 400 km before splashing into the sea.
The Sunday exercise appeared to be another response to Seoul's test launch of a homegrown solid-propellant space rocket on Friday, Yonhap reported citing North Korean observers.
The US Indo-Pacific Command issuing a statement said it is aware of the ballistic missile launch which highlights the destabilising impact of North Korea's ballistic missile programs.
"While we have assessed that this event does not pose an immediate threat to US personnel or territory, or to our allies, the missile launch, alongside the three launches conducted less than 24 hours ago, highlights the destabilizing impact of the DPRK's unlawful WMD and ballistic missile programs," the statement posted on its Twitter handle read.
The US Indo-Pacific Command said US commitments to the defence of the Republic of Korea and Japan remain ironclad.
(with inputs from ANI)