China is North Korea's longtime ally and economic benefactor, their relationship forged in the bloodshed of the Korean War, when Mao Zedong sent millions of "volunteers" to fight US-led United Nations forces to a standstill.

The two countries signed a treaty of friendship, cooperation and mutual assistance in the event of an armed attack on July 11, 1961, with Mao describing the allies as close as "lips and teeth".

Relations have fluctuated over the years due to Pyongyang's growing nuclear ambitions, but with negotiations between North Korea and the US at a standstill, both sides have moved to strengthen their alliance.

"Despite the unprecedentedly complicated international situation in recent years the comradely trust and militant friendship between the DPRK and China get stronger day by day," Kim wrote in his message to Xi, referring to the North by its official name.

In the message carried by the KCNA news agency, Kim highlighted the role of the pact in "ensuring peace and stability in Asia and the rest of the world now that the hostile forces become more desperate in their challenge and obstructive moves".

Xi wrote he planned to bring "greater happiness" to the two countries and their people "by steadily leading the relations of friendship and cooperation between the two countries to a new stage," KCNA said.

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