Chinese President Xi Jinping and Russian President Vladimir Putin vowed to strengthen ties with North Korea in congratulatory letters sent to North Korean leader Kim Jong Un on the country's 78th founding anniversary, state-run media said Wednesday. Xi praised a series of "development successes" in North Korea spurred by Kim's focus on "economic and social construction," the official Korean Central News Agency reported. "It is the consistent and steadfast policy of the Chinese party and government to successfully defend, consolidate and develop the China-DPRK relations," Xi wrote, according to KCNA. The Democratic People's Republic of Korea is the official name of North Korea. Xi said his visit to Pyongyang in June opened up a "new chapter" in relations between the two countries and vowed to work together to "accelerate the development of each side's party and state affairs." China has long been North Korea's largest trading partner, and international observers say it continues to help Pyongyang skirt punishing economic sanctions. Ties had appeared to cool in recent years, however, as Nort