Yang Hyong Sop, a top official in North Korea's ruling party, made the comments to broadcaster APTN in an exclusive interview Friday in the North Korean capital.
Kim Jong Un made his public debut last week after being promoted to four-star general and vice chairman of the ruling Workers' Party of Korea's central military commission. That confirmed analyst speculation that he had been chosen to succeed his ailing father as eventual ruler of the impoverished authoritarian regime that pursues active nuclear and missile programs.
North Korea's succession is being closely watched because of concerns over Kim Jong Il's health. The 68-year-old is believed to have suffered a stroke in 2008 and regional powers are concerned about possible instability in the country were he to die without having a firm succession plan in place.
Kim Jong Il took over as North Korea's leader in 1994 when his father, national founder Kim Il Sung, died of heart failure in what became the first hereditary succession in the communist world.
Tuesday's Korean-language report did not specify exactly when the visit took place. Also attending the event were top military officers and other ruling party officials including Jang Song Thaek and his wife, Kim Kyong Hui, who is Kim Jong Il's younger sister.
Jang and Kim Kyong Hui are considered close to Kim Jong Il and are expected to play a key role in ensuring the succession process goes smoothly.
Kim Jong Un was only mentioned in the report as being among the accompanying officials. He was listed third after North Korean Premier Choe Yong Rim and Chief of the General Staff of the Korean People's Army Ri Yong Ho.
Kim Jong Il expressed "great satisfaction over the fact that all the units are fully prepared to beat back in time any surprise enemy invasion and defend the socialist homeland as firm as a rock," KCNA said in a later English-language version of the report.
He also had a photo session with participating army commanders and members of the unit, identified by its numerical designation of 851, KCNA said. The drill was held to commemorate the 65th anniversary of the ruling party, which is to be marked Sunday.
Late Tuesday, North Korean state television broadcast still photos of the drills and of Kim Jong Il shown with others who accompanied him. Kim Jong Un was visible in one of the photos standing two people away from his father in a group picture.
The world got its first glimpse of Kim Jong Un last Thursday, two days after North Korea held its biggest Workers' Party gathering in 30 years. The younger Kim, believed to be in his late 20s, was shown in several group photos taken with his father and other party and military officials and published in the country's main Rodong Sinmun newspaper.
He also was seen Thursday in video footage of the party meeting broadcast on state television in which he was shown applauding with other delegates during a speech lauding his father.
Separately, the South Korean military is preparing to immediately resume full-scale propaganda activities against North Korea in the event of "any new provocations" by the country, Defense Minister Kim Tae-young said Tuesday.
"We plan to put our preparations into action when a political judgment sees the need for us to further pressure the North," he told lawmakers. He spoke a day after saying that Pyongyang may launch provocative acts as it carries out its leadership succession plan and to disrupt a summit of the Group of 20 nations next month in Seoul.
Kim's comments also came after North Korea said last week it might fire artillery at sites in the South from where leaflets criticizing the country are launched by civilian activists using balloon across the heavily fortified border.
In 2004, the rival Koreas ended decades of propaganda campaigns as relations warmed following a landmark summit in 2000. South Korea, however, installed a dozen propaganda loudspeakers along the border as a part of punitive steps taken against the North over the March sinking of a South Korean warship in which 46 sailors died. They have yet to start broadcasting from the speakers.
A multinational investigation led by Seoul concluded in May that a torpedo fired from a North Korean submarine sank the 1,200-ton warship. North Korea has denied involvement.