North Korean leader Kim Jong-il travelled to China on Tuesday in a special armoured train, reports said.
The North Korean leader rarely leaves the secretive state
The train carrying the secretive leader passed through the town of Dandong on the Chinese border before dawn amid tight security, the reports said.
"The scene was quite similar to one in April 2004, when Kim Jong-il visited China by a special train," said South Korean news agency Yonhap.
Neither North Korea or China, its closest ally, have confirmed the visit.
A Chinese foreign ministry spokesman said he had "no specific information" about the trip.
South Korea's National Intelligence Agency said it was checking the validity of the report, as was the defence ministry.
The reported visit comes as six-nation talks on North Korea's nuclear ambitions appear to be stalled.
North Korea agreed at six-party talks in Beijing in September to abandon a nuclear programme in return for economic and security guarantees.
But soon after, it said it would not scrap its nuclear deterrent until it was given a civilian nuclear reactor.
Both Japan and the US have rejected Pyongyang's demand for a reactor.
China, which has hosted the talks and is seen as the only country with influence over North Korea, is keen for the talks to succeed.
Kim Jong-il, who has ruled the Stalinist regime since the death of his father Kim Il-sung in 1994, rarely travels abroad. When he does so he refuses to fly, instead travelling by a special armoured train.