"A decision was made to award the title of Marshal of the DPRK (North Korea) to Kim Jong-un, supreme commander of the Korean People's Army," the Korean Central News Agency (KCNA) said in a brief report.
The announcement followed North Korea stating Monday that army chief Ri Yong-ho had been removed from all his posts because of "illness" and a little-known general, Hyun Yong-chol, had been promoted to the rank of vice-marshal.
One hour before the report, the KCNA indicated it would make an "important announcement" at noon.
The abrupt departure of Ri is being interpreted by senior officials in Seoul as the first political purge by Kim Jong-un, who is believed to be reconfiguring the North's power structure in order to solidify the power he inherited following the death of his father Kim Jong-il last December.
The promotion indicates another step by the North to equip Kim with the top titles the young leader needs for strong leadership, analysts said.
"Kim Jong-un was given the title he deserves as the firmly-seated North Korean leader and the supreme commander," said Paik Hak-soon, a senior researcher at the Sejong Institute. "The title awarding came at a time Kim is solidifying his power."
Kim was on track to assume the title, Paik said, adding the decision may have been made in conjunction with the promotion of Hyun to the position of vice-marshal.
The appointment, which came only two years after Kim was awarded the title of a four-star general in September 2010, is also seen to assert his domination over his reclusive country as well as the military, a government official said, on condition of anonymity.
"The latest action is very symbolic," the official said. "It is meaningful in that Kim is firming his grip over the entire North (through the advancement) following the dismissal of Ri Yong-ho."
For now, the decision seems like only an internal matter, a separate official said. "(Late leader) Kim Jong-il was also awarded the title one year after becoming supreme commander."
The South detected no unusual moves from the North's army following the announcement, another government source said.
In an early-morning security meeting, South Korean President Lee Myung-bak instructed top defense, intelligence and foreign-affairs officials to keep a close watch on North Korea and to work closely with other nations regarding the recent personnel shifts.
"Since the sudden dismissal of an influential military figure in the North is not something usual, (the president) received reports on the latest news of the North's power circle and the military," an official from the presidential office said.
North Korean defectors to the South said Ri's dismissal came after he was rumored to have fallen out of favor with Kim, apparently for an attempt to deceive the young leader.
"A rumor is circulating among North Korean soldiers that Ri Yong-ho stepped down on account of trying to fool Kim Jong-un," a North Korean defector said. "When Kim visited Tank Division 105 on the first day of the new year, the division had a group of malnourished soldiers segregated out of sight."
After an unscheduled encounter with the malnourished soldiers, Kim rebuked Ri for attempting to cover up the realities of the military, the defector said, quoting his son who now serves in the North Korean army.
After the New Year's Day incident, speculations circulated that Ri might be replaced, the defector said. Kim later instructed officials to feed soldiers well, he said.
A government official declined to confirm that rumor, saying, "It's hard to confirm although there are several other speculations about the dismissal."