Local police had detained 12 people, including the owner of the exam training centre on an upper floor, Xinhua said on Thursday morning.
Hours after the Xinyu fire, Xi urged authorities to “find out the cause as soon as possible, seriously investigate it in accordance with the law, and make a deep reflection”, according to Xinhua.
“[The authorities] should curb the trend of repeated and consecutive occurrences of various safety accidents,” Xi said on Wednesday night, while urging local governments to help ensure social stability.
Xu Hong, the mayor of Xinyu, a city of 1.2 million, said that the blaze was sparked after renovation workers “illegally lit a fire” in the basement of the building. The fire then spread to street-level shops and a hotel on the first floor, as well as the exam training centre.
“We apologise to the families of the victims, the injured and their families,” Xu said on Thursday morning.
Eyewitnesses quoted by mainland media said classes were being held on the first floor when the fire broke out on Wednesday afternoon, with flames blocking the only exit staircase. Videos circulating on social media showed several people jumping out of windows to escape the blaze.
The building is in an old residential area, with no property managers in charge, local media said.
The second deadly fire within a week sparked widespread concern online, with several fire-related hashtags trending on Chinese social media platform Weibo.
Multiple trending posts questioned whether safety checks in Xinyu were adequate.
Web users also raised questions about the infrastructure and fire prevention capabilities of the stricken school in Henan. Local police later detained seven staff.
The back-to-back blazes occurred just two months after another deadly fire that drew comment from Xi.
Xi had then also urged officials to identify risks and improve emergency plans to “curb major accidents”.
Following the Yunnan landslide, he called on local bodies to be alert for natural disasters, traffic accidents and work safety incidents amid a cold wave sweeping southern China ahead of next month’s Lunar New Year travel period, so as to “effectively ensure the safety of people’s lives and property”.