The rail trip, which has not been confirmed by either country, dashed hopes of a meeting with former US president Jimmy Carter who is on a mission to North Korea to try to win the release of a jailed American.
South Korea's Yonhap news agency said Kim might be accompanied by the rarely-seen Kim Jong-un, 27, who is expected to be named to the ruling North Korean party leadership at a rare meeting next month.
Speculation about succession has intensified since Kim Jong-il, now 68, suffered a stroke in August 2008, but he has since recovered sufficiently to work.
"Kim may have decided to tackle this issue in person as China has yet to reach an understanding about the succession at a time when the nuclear issue has not yet been resolved," Professor Yang Moo-Jin of the University of North Korean Studies told AFP.
Chung Seong-Chang of the Sejong Institute said Kim's visit underscored Pyongyang's desperate need for food aid and construction materials from China.
"Ahead of the party convention, Kim has to ease complaints over food shortages and stabilise people's livelihood," he said, adding Kim wants to make sure the September meeting in a festive mood.
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