Various South Korean media outlets raised the speculation after reports that an unconfirmed North Korean official had arrived in Beijing late Monday on a train.
A senior South Korean official told reporters Tuesday at the Presidential Office that Seoul is keeping close watch as a top-ranking North Korean official appears to be visiting China but have not confirmed who was on the train.
While some observers believe Kim himself is making a secret trip to the neighboring country, others suggest the visitor may be his younger sister Kim Yo-jong as she would have preferred to travel to Beijing by train due to her pregnancy.
The younger Kim visited South Korea last month, leading a high-level delegation to the Pyeongchang Winter Olympic Games.
Experts say Beijing and Pyongyang both wish to strengthen their ties before the North Korean leader begins negotiations with Seoul and Washington.
The North Korean leader's late father Kim Jong Il had visited China in May, 2000, weeks ahead of the first inter-Korean summit in June.
China and the North's relations cooled substantially after Pyongyang continued to develop its nuclear and missile program which led to Beijing intensifying its sanctions on the regime.
"If the U.S.-North Korea summit goes well then sanctions on North Korea would be eased but if negotiations fail then sanctions on the regime may be strengthened. But what ultimately matters for the North is how China would respond," said Koh Yu-hwan, a professor of North Korean studies at Dongguk University.
Meanwhile, experts say China is likely aiming to bolster ties with the North, to retain its influence on the regime amid the rapidly changing atmosphere on the Korean Peninsula.
The presidential official said it is hard to confirm the details of the high-ranking official's visit, due to diplomatic ties between Beijing and Pyongyang, but Seoul is communicating with Washington on the situation, Yonhap reported.