Chinese President Xi Jinping and Russian President Vladimir Putin have sent congratulatory letters to North Korean leader Kim Jong-un marking the North's national founding anniversary, vowing to strengthen "strategic" relations with Pyongyang, North Korea's state media said Wednesday. The letters arrived as North Korea observed the 78th anniversary of its national founding on Sept. 9, 1948, by the current leader's late grandfather, Kim Il-sung, according to the Korean Central News Agency (KCNA). Xi noted that Kim has achieved "a series of development successes" in recent years by focusing on "economic and social construction." "It is the consistent and steadfast policy of the Chinese party and government to successfully defend, consolidate and develop the China-DPRK relations," he wrote. DPRK is the acronym for North Korea's official name, the Democratic People's Republic of Korea. Referring to his visit to Pyongyang in June, Xi said the occasion opened up "a new chapter" in the development of the bilateral ties, vowing to work with Kim to accelerate the development of each side's party