Kim expressed his "deep" thanks to Putin in a reply to the Russian president's recent congratulatory message marking North Korea's 78th founding anniversary that fell on Sept. 9, according to Pyongyang’s official Korean Central News Agency (KCNA).

North Korea's will remains "firm without doubt to expand and deepen the joint work with the Russian Federation in a comprehensive way (...)  based on the tradition of friendship, good neighborliness and mutual aids and the firm relations of alliance between the two countries," the letter showed.

He did not specify what the "joint work" meant, but it appeared to point to deepening military and economic cooperation between North Korea and Russia.

The North's leader said the two countries were continuously "writing a new biography of the great history of alliance" in the face of an "uneven historical storm" and promised unwavering support for Russia, the KCNA showed.