They landed here on Sunday afternoon after travelling to the US capital.

Kim, Pyongyang's lead negotiator, had talks with Secretary of State Mike Pompeo and met with President Donald Trump during his three-day stay there.

He had been widely expected to return to Pyongyang on Tuesday via a North Korean flagship carrier.

Air Koryo operates flights between Pyongyang and Beijing on Tuesdays, Thursdays and Saturdays.

But he opted for a Chinese-flagged plane departing a day earlier.

"It seems attributable to the urgency of reporting to Kim Jong-un face-to-face," a source here said.

The North Korean leader is slated to hold another summit with Trump late next month, according to the US government. No date or venue has been announced yet. (Yonhap)