Seoul, Sep 25 : North Korean leader Kim Jong-un has said his country's strong relations with China will further grow against "vicious challenges and obstruction by the hostile forces" in a letter to Chinese President Xi Jinping, Pyongyang's state media reported on Saturday. According to Pyongyang's the Korean Central News Agency (KCNA), Kim made the remarks in a reply to Xi's earlier letter that offered congratulations on the North's 73rd founding anniversary, where the Chinese leader said he intends to "develop these ties of friendship and cooperation on a long-term basis and in a stable way", Yonhap News Agency reported. "I am very pleased to see the comradely unity and cooperation between North Korea and China steadily growing strong while smashing the vicious challenges and obstruction by the hostile forces," Kim said in the letter on Wednesday, adding their relations will "further develop day by day". North Korea has strengthened relations with other socialist countries, including China and Russia, amid a prolonged stalemate in denuclearization talks with the US. On Friday, Kim's sister, Kim Yo-jong, said South Korean President Moon Jae-in's recent proposal to declare a formal end to the 1950-53 Korean War is an "admirable idea" and that Pyongyang is willing to discuss improving inter-Korean ties if Seoul removes what she called "inveterate hostile policy" towards the North' /IANS