Xi sent congratulations to North Korean leader Kim Jong-un on the 77th anniversary of North Korea's founding, state media KCNA said.
"The Chinese side is ready to join hands in promoting the China-DPRK friendship and the socialist cause of the two countries through the intensified strategic communication and brisk visits and close co-operation with the DPRK side," KCNA quoted Xi's message as saying, referring to North Korea's official name.
The leaders of the two nuclear-armed neighbours met in Beijing last week when Kim attended a military parade there on a rare trip to a multilateral gathering.
At their first meeting in six years, Xi called for closer co-ordination with North Korea in global and regional affairs.
Kim's Beijing visit would help North Korea's relations with China, which have frayed as Kim cultivated Russian ties, and help Pyongyang secure economic aid in the face of continued sanctions, experts say.
He was seen walking and talking with Xi and Russian President Vladimir Putin during the massive event which China used as a portrayal of force.
After returning from Beijing, Kim inspected a ground test of a "high-thrust solid-fuel engine" made in carbon fibre, in what he called a "significant change" in expanding the nuclear strategic forces, state media said on Tuesday.
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