On Friday, China said the two countries pledged to strengthen strategic communications "at all levels" and reaffirmed "unswerving stance" on deepening ties as Sun met his North Korean counterpart, Pak Myong Ho, in Pyongyang.
Sun arrived in Pyongyang on Thursday, passing through the city of Sinuiju on the border of the two countries. It was the latest exchange by North Korea with China and Russia while it steps up confrontation with the United States and South Korea.
North Korea has dramatically upgraded its ties with the two U.N. Security Council permanent members and traditional allies, which have come around to stand with Pyongyang as it speeds up the development of an array of tactical and strategic weapons.
Beijing and Moscow, which previously voted against Pyongyang at the U.N. for its ballistic missile and nuclear programmes, have since 2017 refused to join in imposing more sanctions, backing the North's position it is exercising its right to self defense.
(Reporting by Jack KimEditing by Chris Reese and Sandra Maler)
