North Korea leader Kim Jong Un and Russian President Vladimir Putin exchanged letters pledging to develop ties into what Kim called a “long-standing strategic relationship”.

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 on Tuesday, 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”, state news agency 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.