North Korean leader Kim Jong-un has planted memorial trees with key ruling party officials in Pyongyang to mark his country's tree-planting day, Pyongyang's state media said Thursday.

On Wednesday, Kim attended the event with participants of the recently concluded Conference of Secretaries of Primary Committees of the Workers' Party of Korea, where they reviewed progress in key party tasks and pledged to improve them.

During the event, Kim renewed calls for unity among party ranks, saying, "As the trees with firm roots can weather any storm, the party that took its deep roots among the people and depends on them is invincible," according to the North's official Korean Central News Agency (KCNA).

"(Kim) called on the secretaries of the primary Party committees to make redoubled efforts to turn our Party into a united group strong in its fighting efficiency that formed a harmonious whole with the popular masses," it added.

Kim was accompanied by top officials, including his sister, Kim Yo-jong, and Jo Yong-won, secretary for Organizational Affairs of the party's Central Committee.

The North is known to be struggling with severe deforestation caused by excessive logging for fuel. (Yonhap)