North Korean leader Kim Jong-il is paying his second visit to China within a year, according to reports from South Korean media.
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 BBC reports.
The reclusive leader, who rarely travels abroad, last visited China in May. It is not yet clear how long Mr Kim will remain in China or what the main purpose of his visit might be.
The visit comes amid renewed speculation about a successor to Kim Jong-il, who is believed to have suffered a stroke two years ago. He is thought to be manoeuvring to make his third son, Kim Jong-un, his heir, the BBC reports.