North Korean Premier Pak Thae-song has vowed efforts to expand bilateral ties with China in a thank-you message to Chinese Premier Li Qiang, as he has returned home after a three-day trip to China for a key treaty anniversary, state media reported Monday. A North Korean delegation led by Premier Pak returned to Pyongyang the previous day from China, where he attended an event to mark the 65th anniversary of the signing of the two country's frien…