(CNN) -- North Korean leader Kim Jong Il is visiting China, the South Korean JoongAng Daily reported Thursday, citing a South Korean government official.

"[North Korea's] National Defense Commission chairman Kim was detected to be visiting China on his private train," said the official.

The official said Seoul did not understand the purpose of the visit nor did the government know whom Kim planned to contact.

The government-funded Yonhap News Agency reported that Kim "appears to be traveling to China" and cited a senior South Korean government official.

"Signs have been detected that Chairman Kim visited China early Thursday morning," the official said on the condition of anonymity. "We are still trying to grasp his exact destination and purpose of the visit."

Kim spent five days in China in May and had a summit with Chinese President Hu Jintao.

The Korea Herald, citing television station YTN, said that Kim might be visiting with top Chinese officials to discuss handing power to his youngest son, Jong-un. Kim is accompanied by his son, YTN reported, quoting a South Korean government official.

Kim's reported visit abroad coincides with a visit to North Korea by former U.S. President Jimmy Carter, who is seeking the release of a U.S. citizen detained for illegally entering the country.

"Carter is known not to have met Kim on Wednesday, the first day he spent in the North," JoongAng Daily reported.