The train carrying the reclusive North Korean leader passed through the Chinese border town of Dandong amid tight security before dawn on Tuesday, reports quoted unidentified sources as saying.
South Korea's National Intelligence Agency told AFP it was checking the report while the defence ministry, quoting military intelligence, said it had no information on the matter.
North Korean and Chinese media carried no reports of a visit by Kim, who rarely travels abroad.
If confirmed that this would be Kim's fourth trip to China since May 2000. The reclusive leader last visited in April 2004.
Last September, China hosted six-party talks aimed at ending the standoff over North Korea's nuclear weapons programme, and President Hu Jintao visited Pyongyang in October to discuss the stalled talks.
Under an accord reached at the talks, the Stalinist country had agreed to eliminate its nuclear weapons in return for diplomatic and economic benefits.
But the last round of talks in November ended in stalemate after North Korea demanded that the United States lift sanctions imposed on its firms.
China's President Hu Jintao(R)
visited Pyongyang in October
The US Treasury Department in September had told US financial institutions to stop dealing with a Macau bank, Banco Delta Asia, which it accused of being a front for North Korean counterfeiting.
A month later the US blacklisted eight North Korean companies allegedly involved in the spread of "weapons of mass destruction."
Pyongyang denies the charges and insists that the sanctions, targeting the flow of hard currency to North Korea, are a major roadblock to six-party talks.
Washington insists sanctions are unrelated to the nuclear standoff which erupted in October 2002 over US charges that North Korea was seeking to build nuclear weapons through a secret uranium enrichment programme.