China is Pyongyang's sole major ally and North Korean leader Kim Jong Un travelled across the neighbouring country by train to his second meeting with US President Donald Trump, which ended ahead of schedule Thursday without any joint announcements.

Chinese foreign ministry spokesman Lu Kang said he had yet to hear what Trump or the North Korean leader had to say about the meeting.

"But I have always hoped that everyone can realise that the nuclear problem on the Korean peninsula has been going on for many years, and that solving this problem is definitely not something that can be achieved overnight," Lu said at a regular press briefing.

"It is not a very easy process, otherwise it would not have dragged on until today," Lu said.

Lu reiterated Beijing's call for Washington and Pyongyang to "take care of each other's legitimate concerns."

"I hope that both sides can continue to maintain such a dialogue," he said.

Kim met with Chinese President Xi Jinping four times in the past year, most recently in January.

The meetings were seen as opportunities to brief each other before and after Kim's meetings with Trump and South Korean President Moon Jae-in.

It is unclear whether the North Korean leader will see Xi when he takes the train back across China to return home as the route is a tightly-guarded secret.

China is North Korea's main trade partner, but it has backed a raft of UN sanctions following Pyongyang's nuclear and missile tests.

North Korean deputy foreign minister Ri Kil Song met with Chinese Foreign Minister Wang Yi on Thursday in Beijing.

"This is the right time to come," Wang said.

"We can discuss how to take the 70th anniversary of the establishment of diplomatic relations between China and the DPRK as an opportunity to promote the healthy and smooth development of bilateral relations," he added, using the acronym for the North's official name.

Ri said the purpose of his delegation's visit was to "put into practice the important consensus reached during the meetings" between Xi and Kim "and to further strengthen communication and exchanges between the two foreign ministries."