Coinciding with the unconfirmed trip, former US President Jimmy Carter left Pyongyang on Friday with an American, Aijalon Mahli Gomes, who was arrested in January and sentenced to eight years of hard labor for illegally entering North Korea.
There was no indication that Carter met Kim. State media in the North said number two leader Kim Yong-nam had told Carter that Pyongyang was committed to denuclearizing the peninsula and resuming stalled talks on its nuclear disarmament.
"The Pyongyang visit paid by Jimmy Carter, ex-president of the US, provided a favorable occasion of deepening the understanding and building confidence between the two countries," the North's official KCNA news agency said.
There have been no firm sightings of the 68-year-old Kim Jong-il in China, who has appeared frail and gaunt since suffering a stroke in 2008.
A hotel in the northeast city of Jilin was under heavy police guard on Friday morning, a practice seen with previous visits and a clue he may have spent the night there.
A motorcade with more than 30 black cars and a military truck later swept out of the hotel and headed to Changchun, capital of Jilin province, where Jilin city is also located.
South Korean media reported that Kim meet Chinese President Hu Jintao in Changchun.
"It is highly likely Kim mentioned economic cooperation to fight economic difficulties caused by recent flooding, as well as airing the succession issue of his third son, Kim Jong-un," the Korea Economic Daily quoted an unnamed South Korean government official as saying.
By passing up the publicity of a meeting with Carter and instead apparently visiting China, Kim showed how much store he placed in ties with Beijing, said John Park, an expert on North Korea at the United States Institute of Peace in Washington.
"It would be propaganda coup for Kim Jong-il to meet with Jimmy Carter, and for him to pass that up, one wonders what the Chinese are offering," said Park.
South Korean officials have said Kim appeared to have been in northeast China since Thursday, possibly accompanied by Jong-un who may be his heir apparent.
Kim may be seeking approval for succession plans from China, his beleaguered country's key economic and political backer, but Beijing is unlikely to shed any details as the two countries are deeply secretive about their dealings.
Changchun alone has an economy that far outweighs North Korea's. In 2009, the city of more than seven million had a GDP of $43 billion, compared to $24 billion, the size of the North Korean GDP as estimated by the South Korean central bank.
There have been heightened tensions on the Korean peninsula after the March torpedoing of a South Korean navy ship, blamed by Seoul on the North. Pyongyang denies sinking the ship.
South Korea and the US have said resuming the six-party nuclear disarmament talks will be impossible until the ship sinking dispute is settled.
The Workers' Party (WPK), which rubber-stamps major policy decisions in the secretive North, is due to hold a rare meeting in September at which the assembly could set the succession issue in motion, analysts say.
Many analysts say Kim appears in feeble health, and some say he may be hastening his son's succession to the dynasty that has ruled North Korea since its founding after World War Two.
"With the big party meeting in September, you have what many people believe is Kim Jong-il's last shot at passing the baton," said Park.
China has only officially confirmed Kim's previous visits after he left, and there have been no reports this time in either country's state-controlled media.
It would be the second time since May that Kim has visited China.
