The suspected visit comes amid increasing speculation over who is to succeed the ageing Kim, who has suffered poor health in recent years.
South Korean television also reported that Kim, 68, might be accompanied by Kim Joug-Un, his youngest son and presumed political heir.
Beijing has also recently attempted to revive international efforts to disarm North Korea of its nuclear weapons.
Reports from North Korea have also stated that the country is to receive an unspecified amount of "emergency relief materials" from China amid a worsening food crisis and flooding in the north of the country.
"This measure will encourage the Korean people in their efforts to recover from the flood damage as early as possible and more energetically step up the building of a thriving nation," the state Korean Central News Agency (KCNA) said late Wednesday.
A diplomatic source in Beijing was quoted by Yonhap that Kim Jong-Il's special train travelled north and passed Jian City near the border with North Korea.
"This means Beijing may be not his first destination. He may be heading for Jilin City," he said.
China has so far not commented on the reports. The two countries have previously made it clear that they will not confirm any trips by the North Korean leader until they are completed.
Analysts have said that the apparent visit comes ahead of a key party meeting in North Korea, in which leaders are to be elected including Kim Joug-Un.
Kim made a five-day visit to China in May, for a meeting with Hu Jintao, the Chinese president.