SHANGHAI, Dec 18 (Reuters) - ** China stocks rose marginally on Friday, driven by gains in coal miners on reports of worsening electricity shortages in some provinces. ** At the midday break, the Shanghai Composite index was up 0.13% at 3,409.31, while China’s blue-chip CSI300 index was up 0.03%. ** The smaller Shenzhen index was up 0.15%, the start-up board ChiNext Composite index was higher by 0.34% and Shanghai’s tech-focused STAR50 index was up 0.49%. ** Some Chinese cities reported increase in electricity shortages, boosting shares including Shanghai Datun Energy Resources Co Ltd, Datong Coal Industry Co Ltd and Anhui Hengyuan Coal Industry and Electricity Power Co Ltd. All of them hit their daily upper price limit. ** The state planner was quoted by official Xinhua news agency saying power usage was higher on rapid recovery of industrial production and extreme cold weather in some provinces. It said it would take measures to ensure electricity supply. ** But gains were capped by wories about Sino-U.S. relations. The U.S. energy secretary signed an order prohibiting electric utilities that supply critical defense facilities from importing certain power system items from China. ** Chinese H-shares listed in Hong Kong fell 0.49% to 10,494.11, while the Hang Seng Index was down 0.67% at 26,499.90. ** Around the region, MSCI’s Asia ex-Japan stock index was firmer by 0.66% while Japan’s Nikkei index was down 0.12%. ** The largest percentage gainers on the main Shanghai Composite index were Ningbo Thermal Power Co Ltd, up 10.15%, followed by Shanghai Datun Energy Resources Co Ltd, gaining 10.06%, and Dongfeng Electronic Technology Co Ltd , up by 10.04%. ** So far this year, the Shanghai stock index is up 11.8% and the CSI300 has risen 22.5%, while China’s H-share index listed in Hong Kong is down 6%. Shanghai stocks have risen 0.52% this month. (Reporting by the Shanghai Newsroom; editing by Uttaresh.V)