Photos from China showed a man who looked like Mr Kim in Dalian
North Korean leader Kim Jong-il is making a rare visit to China, reports and images from the region suggest.
His personal train was reported to have crossed into China at the border city of Dandong early on Monday on the way to the port city of Dalian.
Images then showed a man who looked like Mr Kim leaving a hotel in Dalian.
The South Korean government said it could not confirm the reports - but there have been rumours of such a visit for several weeks.
Mr Kim habitually travels by train and his visits are generally not confirmed until he has returned to North Korea.
He is not believed to have been abroad since a 2006 visit to China. China is North Korea's main trading partner and the country perceived to have the most influence on the communist state.
The train thought to be carrying Kim Jong-il arrived in Dandong early on Monday, South Korea's Yonhap news agency said, citing an unidentified government official.
"We have confirmed the arrival of a special train at Dandong, and we believe it is highly likely that Chairman Kim is on board," the official said.
About 200 Chinese police officers guarded the station and blocked surrounding roads ahead of the train's arrival.
Yonhap said that the 17-carriage train then went to Dalian, where a convoy of limousines was later seen arriving at a top hotel.
Images then emerged of a man who looked like the North Korean leader getting into a car in Dalian.
It is not yet known whether Mr Kim will head to Beijing for talks with Chinese leaders.
The possible visit comes amid growing international concern over North Korea.
Analysts suggest that Mr Kim could be seeking economic aid from Beijing in the wake of a disastrous currency reform last year that worsened inflation.
China could urge North Korea to return to six-party talks on ending its nuclear ambitions - talks which have been stalled for a year.
But South Korea has said that the talks could not restart if evidence emerges that North Korea sank the Cheonan warship.
It went down on 26 March near the disputed inter-Korean maritime border, after an unexplained explosion. Forty-six South Korean sailors were killed.
The BBC's John Sudworth: "Another indication of the deeply clandestine and secretive world of the N Korean leader"