The eldest son of North Korea's late leader Kim Jong-il was spotted at an airport in Beijing last week, a South Korean professor said Monday, one month after his father died of a heart attack in the North.

Kim Jong-nam, believed to be in his early 40s, was sitting in front of the departure gate for a flight to Macao Saturday, sporting jeans, a dark blue padded jacket and a baseball cap, said Park Seung-jun, a professor of Chinese Language and Literature at South Korea's Incheon University.

"He looked like Kim Jong-nam, so I asked him, 'Aren't you Mr. Kim Jong-nam?' and to my surprise, he stood up from his seat and said yes," Park told Yonhap News Agency by phone.

Asked whether he was shocked by his father's sudden death on Dec. 17, Kim replied, "Oh...(that's) nature," Park said.

The professor said he ran into Kim while walking through Terminal 3 of Beijing Capital International Airport on his way to board a flight to Seoul. Kim did not appear to have any escorts and seemed to have put on weight since the last time he was photographed by the press, Park said.

Responding to Park's remarks that Kim, as the eldest child, may have to take good care of his younger siblings, Kim replied, "Well...I guess so."

Kim Jong-nam is known to have two younger brothers and a younger sister, including his 20-something-year-old half-sibling Kim Jong-un, who has taken the helm of Pyongyang in line with their father's intentions.

Asked whether he attended his father's funeral in Pyongyang on Dec. 28, the eldest son did not give a clear answer, Park said.

A source familiar with Kim's activity told Yonhap News Agency last month the eldest son had arrived in Beijing from the Chinese enclave of Macao shortly before the state funeral, spurring speculation he may have been en route to Pyongyang.

Another source familiar with North Korea affairs, however, said it would have been difficult for Kim to attend the funeral under North Korea's political circumstances, as Kim Jong-un consolidates his power in the regime.

Kim Jong-nam has lived abroad for years after apparently falling out of favor with his father for attempting to enter Japan on a fake passport in 2001.

Separately, a Japanese newspaper reported last week that Kim Jong-nam had expressed doubts about his half-brother's hold on power, anticipating the ruling elite would extend its influence over the communist country.

In his first remarks on the North Korean regime since his father's death, Kim told the Tokyo Shimbun in an e-mail sent early this month he had "doubts about how a young successor with some two years (of training as heir) can retain the 37 years of absolute power" wielded by their late father, the newspaper said.

"It is difficult to accept a third-generation succession under a normal reasoning (process)," the paper quoted him as saying. (Yonhap News)