A train believed to be carrying a senior North Korean delegation, which some reports said included the country's leader, Kim Jong-un, left the Chinese capital on Tuesday following a dramatic whirlwind visit.
The conservative South Korea Chosun Ilbo newspaper, citing an unnamed senior intelligence official, said the delegation had included Kim, in what would be his first known overseas trip since taking power in 2011, and that he had since left to return to North Korea.
South Korea's left-leaning Hankyoreh newspaper also reported that Kim had traveled to Beijing for meetings with Chinese President Xi Jinping on Monday afternoon before leaving for a "third location", which could be in China, on Tuesday. It did not cite specific sources.
Beijing has traditionally been the closest ally of secretive and isolated North Korea, but ties have been frayed by North Korea's pursuit of nuclear weapons and China's backing of tough UN sanctions in response.
Kim is due to hold separate summits with China's rivals, South Korea and the United States, and analysts say a visit to Beijing could be preparation for those.
US Defense Secretary Jim Mattis said it "kinda looked like" Kim went to Beijing, but told reporters at the Pentagon, "I don't know."
A senior US official who follows North Korea closely said the available evidence suggested Kim had gone to meet Xi, but stressed that has not been confirmed.
Hong Kong's South China Morning Post said Kim was on the train that left Beijing, citing two anonymous sources.
A Chinese foreign ministry spokeswoman deflected a question 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 they did not know exactly who.
Bloomberg, citing three unidentified sources, reported late on Monday that Kim was in Beijing.
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 at the border city of Dandong.
One source with ties to China's leadership said it was possible the visitor was Kim's younger sister, Kim Yo Jong. She visited South Korea during the Winter Olympics last month, paving the way for a summit between the two Koreas.
The US official, who spoke on condition of anonymity, said it was unlikely Kim Jong-un would have again sent his sister given the importance of a meeting with Xi, which would underscore Kim's standing as a world leader.
South Korean news agency Newsis reported that Kim Yo Jong and the North's ceremonial leader, Kim Yong Nam, were visiting Beijing, citing an unidentified North Korea-related source in Beijing.
'Lot of leverage'
Xi and Kim Jong-un had reasons to meet in advance of Kim's meetings with South Korean President Moon Jae-in in April and possibly US President Donald Trump in May, the US official said.
"Xi has met Trump, and in many respects learned how to deal with him better than some people here do," the official said.
"At the same time, despite the recent tensions, he needs to know what Kim has in mind for dealing with the South and the US, and he still has a lot of leverage with the North."
A senior South Korean official said improving ties between North Korea and China would be a positive sign before the planned summits.
While China, which remains North Korea's biggest trading partner in spite of recent sanctions, has not confirmed any North Korean visit, it has not totally censored speculation.
Posts on Chinese social media have talked about the possibility that Kim Jong-un was in China, some citing family members in Dandong. The rail journey between Dandong and Beijing covers more than 1,100 km (680 miles). It takes at least 14 hours by ordinary service, according to Chinese railway timetables.
Kim Jong-un's father, Kim Jong Il, met then-president Jiang Zemin in China in 2000 before a summit between the two Koreas in June that year. That visit was seen at the time as reaffirmation of close ties with Beijing.
"North Korea likely wants to confirm its relationship with China and believes it has some leverage with which it can ask for things from China," said Yoo Ho-yeol, professor of North Korean studies at Seoul's Korea University.
"If North Korea speaks with the United States on its own, it might feel it is at a disadvantage but, if it has China as an ally, Pyongyang may think it will be able to protect its interests and profits during the summits."