North Korean leader Kim Jong-un has reportedly vowed to promote cooperative relations with China in a letter to Chinese President Xi Jinping.

According to the Rodong Sinmun on Sunday, the official paper of the North’s ruling Workers’ Party, Kim expressed deep gratitude over the fact that Xi sent a message congratulating North Korea on its 75th founding anniversary.

In the letter sent on Thursday, Kim reportedly said that the unwavering support for North Korea by the Chinese party, government and people strongly encourages the North Korean government and its people in their strife to advocate and defend socialism.

Kim then stressed that he believes the two nations will continue “close cooperation” and that their friendly and cooperative relations will steadily develop in conformity with the requirements of the new era and the desires of the two peoples in the future.

Xi sent a congratulatory message to Kim on September 9 on the 75th anniversary of the North’s founding, in which he called China and North Korea “friendly neighbors linked by the same mountain and river.”

Xi also emphasized that no matter how international and regional situations may change, China remains firm in its position to develop its traditional friendly and cooperative relations with North Korea.