SEOUL: North Korea leader Kim Jong Un and Russian President Vladimir Putin exchanged letters on Tuesday pledging to develop their ties into what Kim called a "long-standing strategic relationship," Pyongyang's state media KCNA said.

The letters mark the 78th anniversary of Korea's liberation from Japan's 1910-45 colonial rule, which is also celebrated as a national holiday in South Korea.

In his letter to Putin, Kim said the two countries' friendship was forged in World War II with victory over Japan and is now "fully demonstrating their invincibility and might in the struggle to smash the imperialists' arbitrary practices and hegemony," KCNA said.

"I am firmly convinced that the friendship and solidarity ... will be further developed into a long-standing strategic relationship in conformity with the demand of the new era," Kim was quoted as saying in the letter.

"The two countries will always emerge victorious, strongly supporting and cooperating with each other in the course of achieving their common goal and cause."

The United States has accused North Korea of providing weapons to Russia for its war in Ukraine, including artillery shells, shoulder-fired rockets and missiles. Pyongyang and Moscow have denied any arms transactions.

Last month, Russia's defence minister stood shoulder to shoulder with Kim as they reviewed his newest nuclear-capable missiles and attack drones at a military parade in Pyongyang.

Putin, in his message to Kim, also vowed to bolster bilateral ties.

"I am sure that we will strengthen the bilateral cooperation in all fields for the two peoples' well-being and the firm stability and security of the Korean peninsula and the whole of Northeast Asia," Putin said, according to KCNA.

The leaders of South Korea, the United States and Japan are set to discuss security cooperation over North Korea, Ukraine and other issues at a trilateral summit on Aug. 18 at Camp David.