Quoting reports, the BBC said the scene was similar to one in April 2004, when Kim visited China by a special train.
The trip was not confirmed either by North Korea or China, the BBC report said.
But South Korea's National Intelligence Agency and the defense ministry said they were checking on the reports which come as the six-nation talks on North Korea's nuclear program have not moved forward since September.
At the time, North Korea agreed to abandon the program in exchange for economic and security guarantees. Subsequently, the country insisted on a civilian nuclear reactor but that has been rejected by Japan and the United States, the report said.