North Korean leader Kim Jong-un was part of a secretive delegation that arrived in Beijing by train on Monday and left on Tuesday, according to a source familiar with the matter and foreign media reports.
The visit, which would be Kim's first known journey abroad since he assumed power in 2011, was not announced by North Korea or China and has remained shrouded in mystery.
South Korea said it was closely watching events in Beijing, where a Chinese foreign ministry spokeswoman deflected a question on Tuesday on whether Kim, his sister or some other senior North Korean was visiting.
"At present I have no understanding of the situation you mention. If there is news we will release it," foreign ministry spokeswoman Hua Chunying told a regular daily briefing.
Diplomatic sources in Beijing said a senior North Korean official was in town, but did not know exactly who.
Beijing is the main ally of secretive and isolated North Korea, as well as its biggest trading partner.
The conservative South Korea Chosun Ilbo newspaper, citing an unnamed senior intelligence official, said the delegation had included Kim and that he had left to return to North Korea.
South Korea's left-leaning Hankyoreh newspaper also reported Kim had travelled to Beijing for meetings with Chinese President Xi Jinping on Monday afternoon before leaving for a "third location" on Tuesday. It did not cite specific sources.
The Hankyoreh did not specify where the "third location" was but said it could be in China.
A train believed to be carrying a high-ranking North Korean official departs Beijing station. Source: AAP
The unconfirmed visit came ahead of Kim's planned summit meeting with South Korean President Moon Jae-in in late April and possibly with US President Donald Trump in May.
"The presidential Blue House is watching things in Beijing very closely, while keeping all possibilities open," said the senior official in Seoul, who spoke on condition of anonymity.
Improving ties between China and North Korea, which is pursuing nuclear and missile programs in defiance of UN Security Council resolutions, would be a positive sign before the planned summits, he said.
A Reuters reporter saw a convoy leave Beijing's Diaoyutai State Guest House, where senior foreign leaders often stay, and drive north on Tuesday morning. It was unclear where the convoy was headed.
Later, a Reuters journalist saw what was believed to be the delegation's train pulling out of a Beijing station. The group was reported to have arrived in China on Sunday after crossing from North Korea in the border city of Dandong.
A senior US official who follows North Korea closely said the available evidence suggested that Kim had travelled to Beijing to meet Xi, but stressed that had not been confirmed.
Underscoring the mystery, one senior Beijing-based diplomatic source told Reuters simply: "We just don't know."
One source with ties to China's leadership said it was possible Kim's younger sister, Kim Yo Jong, was in town. She visited South Korea for the Winter Olympics last month, paving the way for a summit between the two Koreas.
