China remained silent on Friday about a reported visit by North Korea's secretive leader Kim Jong-il, with no official word on a trip analysts believe may be to line up Beijing behind his dynastic succession plans.

South Korean officials have said Kim appeared to have been in northeast China since Thursday, possibly accompanied by his youngest son who may also be his heir apparent.

There have been no firm sightings of the distinctive frizzle-haired 68-year-old, who often wears Mao-style safari suits, but a riverside hotel in the northeast city of Jilin was under heavy police guard on Friday morning, a practice seen with previous visits and a clue that he spent the night.

Kim may be seeking China's show of approval for succession plans, but if he is then Beijing is unlikely to shed any details. The two countries are deeply secretive about their dealings.

The Workers' Party (WPK), which rubber stamps major policy decisions in the secretive North, is due to hold a rare meeting in September at which the assembly could set in motion the succession of the leader's son, Kim Jong-un, analysts say.

Many analysts say Kim appears in failing health after a suspected stroke in 2008, and some say he may be in a hurry to secure his son's succession to the dynasty that has ruled North Korea since its founding after World War Two.

China has only officially confirmed Kim's previous visits after he left, and there have no reports this time in the either country's state-controlled media.

It would be the second time since May that Kim has visited, China, which helps prop up North Korea's frail economy.

Kim's reported visit was taking place as former U.S. President Jimmy Carter visited Pyongyang, seeking the release of Aijalon Mahli Gomes, an American sentenced to eight years hard labour earlier this year for illegally entering the country.

(Additional reporting by Jack Kim in Seoul, Tabassum Zakaria in Washington and Chris Buckley in Beijing; Writing by Chris Buckley; Editing by David Fox)