The missive from Pyongyang comes at a time when China is also stepping up diplomacy with Russia -- a day after Russian Prime Minister Dmitry Medvedev reaffirmed bilateral plans to build a "Silk Road on ice" in Beijing on Wednesday.
Russia and China have been North Korea's closest economic partners, although Pyongyang is currently under heavy sanctions, and China may have begun suspending trade with the North.
North Korea's KCNA stated the Chinese leader, who has never met with Kim, wanted to help boost "more wonderful happiness" in the two countries while "defending regional peace."
"I wish that under the new situation the Chinese side would make efforts with the [North Korean] side to promote the relations between the two parties," Xi said, according to KCNA.
North Korea refrained from provocations during China's 19th Communist Party congress last week, to the surprise of many analysts who had said Kim could launch more missiles to anger neighbors.
North Korea had also conducted a missile test in May, when China was hosting the One Belt One Road international forum, an ambitious Chinese initiative that was the focus of discussions between Xi and Medvedev on Wednesday.
Medvedev called the world's second-largest economy a "friendly country" that can work with Moscow to expand "comprehensive strategic interaction," Russia's Tass news agency reported.
The decision to cooperate on a polar sea route in the Arctic Circle comes months after the two sides initially agreed to build an "Ice Silk Road" along the Northern Sea Route in the Arctic, according to China Daily USA.
In the meeting in Beijing on Wednesday, Xi said the two countries should maintain close ties on international issues.
"I would like to reiterate that our course to develop, strengthen and deepen Chinese-Russian relations remains unchanged," Xi said, ahead of U.S. President Donald Trump's first state visit to Beijing next week.
Xi also called on the two countries to "carry out the Northern Sea Route cooperation so as to realize an 'Ice Silk Road,' and to implement various connectivity projects," according to China's Global Times.