South Korean media quoted in London's Guardian newspaper said "satellites have tracked a special North Korean train, the usual form of transport for Mr. Kim, entering Chinese territory."
If true, the visit would be Kim's second this year amid increasing signs of a rift between Beijing and Pyongyang.
It also would come as South Korea acknowledged for the first time it thinks North Korea has nuclear-test capabilities.
"North Korea is believed to have facilities for a nuclear test," the International Herald Tribune quoted Kim Seung Kyu, director of South Korea's National Intelligence Service, telling South Korea's National Assembly in a private meeting.
"The possibility of a nuclear test is always open as soon as Kim Jong Il makes a decision," Kim said, adding his agency had no indication of an imminent test.