TOKYO - Kim Jong-Il's third son, the likely successor to the North Korean leader, has been appointed acting defence chief under his ailing father, a Japanese newspaper reported on Saturday.

Kim Jong-Un started supporting his father as acting chairman of North Korea's National Defence Commission, the evening edition of the Mainichi Shimbun said, quoting unnamed sources close to North Korean leadership.

Kim Jong-Il has ruled the reclusive country, and held its two highest posts of defence commission chairman and secretary general of the Workers' Party of Korea, since his father, Kim Il-Sung, died in 1994.

As acting chairman, which is still considered informal, the 26-year-old son appears to be securing his succession to his father, 67, who reportedly suffered a stroke last August, the newspaper said.

'Mr. Kim Jong-Un appears clearly having been designated to succeed our general (Kim Jong-Il),' one of the sources said, according to the newspaper.

'Mr. Kim Jong-Un is now concentrating on the job of deputy chairman,' the source said. 'If something happens to our general, the chairman's position will be automatically taken by (Jong-Un).'

Little is known about the son, who was believed to have been schooled in Switzerland and was recently reported by South Korean intelligence officials to have been named as the eventual heir to the family dynasty.