North Korean leader Kim Jong Un reaffirmed a pledge to deepen cooperation with Russia in a message to President Vladimir Putin as Pyongyang marked an anniversary of independence from Japan's colonial rule, KCNA state news agency said on Friday.

It was in reply to a message of congratulations from Putin on the Aug. 15 liberation day anniversary where the Russian leader said the bond forged as Soviet soldiers fought against Japan continues to serve as the basis of their ties, KCNA said.

"The friendly feelings of the armies and peoples of the two countries forged and deepened in the bloody struggle against the common enemy serve as a strong driving force for developing ... relations of friendship and cooperation into comprehensive strategic partnership and invincible comradeship," Kim said.

Kim and Putin held a second summit meeting in less than a year in June in Pyongyang, signing a pact on "comprehensive strategic partnership" that includes a mutual defence agreement.

It came amid accusations by South Korea, Ukraine and the United States that Kim is helping Russia in the war against Ukraine by supplying rockets and missiles in return for economic and other military assistance from Moscow.

Kim visited a memorial honoring the Korean revolutionary soldiers who fought in resistance to Japan to end the 1910-1945 colonial rule and the Liberation Tower where Soviet Red Army soldiers are remembered, KCNA said.

North Korea's state founder Kim Il Sung, who is the current leader's grandfather, was backed by Soviet Union General Secretary Joseph Stalin who declared war against Japan near the end of World War Two.

The Soviets backed Kim's communist forces that eventually established North Korea after Korea's liberation in 1945.

North Korean state media made no mention of a blueprint for unification announced by South Korean President Yoon Suk Yeol on Thursday, calling for dialogue with Pyongyang and proposing an international conference on North Korea's human rights.

Coming at one of the lowest points in the two Koreas' ties, Yoon's blueprint was accepted with scepticism among some experts, who doubt whether it is realistic to expect Pyongyang to see it as anything other than an existential threat to its regime.

On Friday, South Korea's Unification Minister Kim Yung-ho, who oversees inter-Korea relations, said he disagreed with those who say North Korea will reject the plan if it responds at all.

"I believe North Korea will carefully review our government proposal," he told a briefing.