North Korean leader Kim Jong Un has travelled to China for a surprise visit to the border town of Dandong, South Korea's official news agency Yonhap reported on Saturday.
Kim is said to have crossed the border via a bridge that was opened a few weeks ago, Yonhap reported, citing an anonymous government source in Seoul.
Neither the Chinese nor the North Korean authorities have so far confirmed the visit.
Kim's trip to China coincides with the anniversary of China's entry into the Korean War, which is known in China as the "War of Resistance Against the US and Aid to Korea."
It has not yet been confirmed whether Kim will take part in any commemorative ceremonies. During the Korean War from 1950-53, Chinese troops fought alongside North Korea against South Korea and US-led United Nations' forces.
There were indications of an imminent visit by high-ranking North Korean officials on Friday, with reports that security measures in Dandong had been stepped up.
China is the most important trading partner of North Korea, which is subject to UN sanctions due to its nuclear weapons programme.
In June, Chinese President Xi Jinping travelled to North Korea on a state visit.
The four-lane, 3-kilometre road bridge across the Yalu River linking Dandong with the North Korean city of Sinuiju was opened in mid-October.
It is intended to facilitate trade between the two countries.