South Korean intelligence sources say they are "100 percent certain" North Korean leader Kim Jong Il traveled by train to China.
Military intelligence officials quoted in media reports also said Mr. Kim's train crossed into the Chinese border town of Dandong before dawn. They said security was heavy, as it had been for a trip by Mr. Kim to China in 2004.
The North Korean leader is believed to dislike flying, and rarely leaves the country over which he exerts absolute control.
Chinese Foreign Ministry spokesman Kong Quan told reporters in Beijing he was not authorized to release any information regarding a possible visit. But he did not deny the visit was under way.
Kong points out China and North Korea are friendly neighbors and maintain a tradition of high-level visits. He also confirmed that Mr. Kim was planning to visit Beijing at some point.
North Korea experts can only speculate at this point about why Mr. Kim may be making a trip. But there is agreement that any discussions will cover Washington's accusations that Pyongyang has been issuing counterfeit U.S. dollars on a large scale.
Analyst Peter Beck, the Northeast Asia director of the research organization the International Crisis Group, says China is facing intense pressure over the counterfeiting allegations - because Chinese banks are North Korea's main point of financial contact with the world.
"So the question is can Washington put enough pressure on Beijing to extend the crackdown on North Korean banking activities. And so this visit by Kim could possibly be designed to make sure that does not happen," said Beck.
The U.S. says it has substantial evidence of Pyongyang's counterfeiting, and it has already put financial sanctions on several North Korean companies. North Korea says it will not return to talks aimed at ending its nuclear weapons programs unless the U.S. withdraws those sanctions - a demand Washington has rejected.
Shin Sang-jin, an international relations professor at Seoul's Gwangwoon University, points out China's political agenda makes this an opportune time for a visit by Mr. Kim.
He says Chinese leaders are about to embark on a new five-year plan for the economy, and the two countries may need to discuss terms for future economic cooperation.
North Korea is heavily dependent on China, its only major ally, for financial and energy support.
Kim Jong Il has visited China four times since 2000. The two countries may choose to handle this possible visit the way they handled Mr. Kim's 2004 visit: they did not confirm the trip had taken place until Mr. Kim had returned to Pyongyang.