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Sept 10 (Reuters) - North Korean leader Kim Jong-il has collapsed and is ill, a South Korean official said on Wednesday, and a U.S. official said he may have suffered a stroke.

Following are profiles of some possible successors to the reclusive leader, who at 66 is known to suffer from diabetes and heart problems and has been the subject of persistent rumours about his health.

THE THREE SONS

Jong-chol is the second oldest of Kim's three known sons and believed to be 26. Educated in Switzerland, he is thought to be suffering from a hormonal disorder but is considered to be the favourite to succeed his father. Despite the lack of any formal grooming for the top job, one North Korea expert said Jong-chol has been accompanying his father on official trips around the country.

Jong-nam, 37, is the oldest but many believe he has fallen from grace for trying to enter Japan with a forged passport.

The youngest, Jong-un, born in 1983, is often cited as the most promising but is seen as unlikely to be picked in a society where the tradition of seniority is so strong.

BROTHER-IN-LAW

Jang Song-taek, 62, is the husband of Kim's sister and a ruling Workers' Party official who appeared to lose favour some years ago in a power struggle with reformers. But more recently, he returned to the inner leadership circle and is often seen as the second-most powerful man after Kim in the ruling party structure, though he officially holds only a relatively low rank.

HEAD OF STATE

Kim Yong-nam, 80, is the head of the North Korean parliament's leadership council and is the country's nominal head of state. While a long-time loyal party member and a former foreign minister, he is not thought to have the legitimacy rooted in the North's revolutionary history, seen as necessary to become leader. If he did succeed Kim, it would likely be as a figurehead.

THE GENERALS

Jo Myong-rok is the head of the powerful Political Department of the North's army and Kim's number two in the National Defence Commission. But he, too, is believed to be in poor health.

Another general, O Kuk-ryol, was named in a 2006 report by the South's intelligence agency as a younger and more likely successor to Kim's military post. The report said he appeared to be a reliable figure who is familiar with South Korea. (Reporting by Jack Kim; Editing by Jonathan Thatcher and Bill Tarrant)

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