North Korean leader Kim Jong-il is paying his second visit to China within a year, South Korean media report.
Yonhap news agency quoted a South Korean official saying that Mr Kim's personal train departed on Wednesday. One report said he was accompanied by his son and possible heir Kim Jong-un.
He visited a school in China's Jilin province, teachers and students said.
North Korean and Chinese officials traditionally do not confirm Mr Kim's visits until his return to Pyongyang.
The reclusive leader, who rarely travels abroad, last visited China in May.
The visit comes amid renewed speculation about a successor to Kim Jong-il, who is believed to have suffered a stroke two years ago.
"Chairman Kim's special train has been confirmed to have left Manpo for China's Jilin around midnight Wednesday," an unnamed South Korean official told Yonhap news agency.
Teachers and a student at Yuwen Middle School in Jilin separately told the media that Mr Kim paid a 20-minute visit to their school on Thursday morning.
His father, Kim Il-sung, reportedly attended the school between 1927 and 1930.
It is not yet clear how long Mr Kim will remain in China or what the main purpose of his visit might be.
Trade could be a motive for the trip, says the BBC's John Sudworth in South Korea's capital, Seoul.
North Korea relies on aid and trade from China to prop up its tottering state-run economy.
But the succession to the ailing Mr Kim could be on the agenda, says our correspondent.
Mr Kim is reported to have had a stroke two years ago and is thought to be manoeuvring to make his third son, Kim Jong-un, his heir.
A major conference of the ruling party of North Korea is to be held in September, and some analysts expect some kind of succession-related announcement there.
One South Korean TV station cited a South Korean official as saying Kim Jong-un was with his father on the trip.
Another possible motive for the trip, say analysts, is discussion of North Korea's nuclear programme.
China has been making moves to resume the six-nation talks on disarming the North, after the sinking of a South Korean warship in March inflamed tensions between Pyongyang and Seoul. North Korea has been blamed for the sinking.
Meanwhile, former US President Jimmy Carter has spent the night in Pyongyang trying to secure the release of a US citizen detained by North Korea since January.
Aijalon Mahli Gomes was sentenced to eight years' hard labour for illegally entering North Korea.
There are reports that he could leave with Mr Carter some time on Thursday.