According to the report by Pyongyang's Korean Central News Agency (KCNA), the letter dated October 1 was sent to congratulate Xi on the 71th founding anniversary of China.
"The Supreme Leader also stressed in the message that he and the party and the people will invariably stand by the General Secretary (Xi), the Communist Party of China (CPC) and the Chinese people in the struggle for accomplishing the joint cause of defending and glorifying socialism gained at the cost of blood," Yonhap News Agency quoted the KCNA report as saying.
"The Supreme Leader in the message expressed his will to strive, together with the general secretary, to ceaselessly consolidate and develop the invincible friendship which has entered a new historical period," it added.
Between 2018 and 2019, Kim and Xi met five times.
The March 25–28, 2018 meeting was Kim Jong-un's first diplomatic trip abroad to meet a foreign leader since coming to power in 2011.