BEIJING—A fire tore through a martial arts training center in China during the early hours of Friday morning, killing 18 people, with one state media outlet reporting that most of the dead were children who were staying there. The blaze broke out at 3 a.m. and injured 16 people, four of whom were in serious condition as reported on the the government of Zhecheng county, in Henan province, website. It was unclear what caused the fire. Most of the casualties were students aged between 7 and 16 staying on the centre’s second floor, the Beijing Youth Daily said in a report that was later deleted from its website. In a social media post, Zhonghong, a news outlet backed by China’s state planner, identified the centre as Zhenxing Martial Arts Centre. Neither Zhenxing or its founder, Chen Lin, answered calls seeking comment. “My kid is nine years old, but I know …