South Korean media reported that Kim's distinctive armoured train was expected to reach Beijing on Tuesday morning, which happens to be Kim's birthday.
The visit comes amid reports that negotiations are under way for a second summit between Mr Kim and US President Donald Trump.
The visits past year were widely seen as a courtesy to China and an opportunity to coordinate strategy ahead of the summits with the USA and South Korean leaders.
A former North Korean diplomat who defected to the South urged a former colleague missing in Italy to come and settle in Seoul on Saturday, as the rare asylum bid makes global headlines.
The reports said a train like the one often used by Kim was seen crossing through the Chinese border city of Dandong late Monday amid heavy security.
Kim said the NIS official said that Jo and his wife left the official residence in early November, weeks before his term was to end in late November.
A special North Korean train has entered China, raising speculation that Kim Jong-un could be on his way to Beijing. Sanctions have been tougher, but North Koreans can buy coal whenever they want if they have money, the source said.
Trump said Sunday the sanctions remain "in full force and effect" and would do so until the United States saw "very positive" results.
Some observers believe that while it is tempting to interpret this "new way" as being a thinly veiled threat of more nuclear or missile tests, it could instead be an implicit warning that the North could further bolster its already improving ties with China if the US fails to ante up.
It said Kim is visiting China at the invitation of Chinese President Xi Jinping.
The former Speaker cited that the leaders of the Republic of Korea and the Democratic Republic of Korea engaged in a series of diplomatic efforts in order to achieve permanent peace in the Korean Peninsula.
While there were no details released about the possible agenda in China, Mr. Kim has been seeking relief from global sanctions, a peace declaration to formally end the 1950-1953 Korean War, and more economic investment.
In recent years, the lack of a reliable power supply led many North Koreans to install cheap household solar panels to charge mobile phones and light their homes.