The French news agency AFP quotes Chinese border officials as confirming that Mr Kim's personal train had crossed the Chinese frontier on Monday. Neither Beijing nor Pyongyang will confirm that Mr Kim is visiting China, but there was no official confirmation of Mr Kim's last visit to China until after he had returned to North Korea. Mr Kim's previous trip to China, in May 2000, was his first known foreign trip since 1983, and he is not thought to have left North Korea again, until now.
China is the isolated country's strongest ally.
There is conflicting evidence about Mr Kim's whereabouts in China. The BBC's Rupert Wingfield Hayes says there has been a mad rush to buy tickets for a special opera performance in Beijing on Tuesday night after rumours that Mr Kim would attend. But an unnamed Western diplomat told the Reuters news agency that there was evidence that Mr Kim had gone to Shanghai, a busy commercial city. That would be consistent with reports from South Korea which suggest that Mr Kim wants to study China's economic reforms and will visit Shanghai and the special economic zone of Shenzhen. North Korea's economy has been cut off from the rest of the world for decades and is in ruins.