Chinese police are holding four suspects after a Shanghai apartment fire that killed 53 people and injured 70 others was blamed on unlicensed welding, state media said.

The fire, which gutted a 28-storey building in China's commercial hub, was sparked by "unlicensed welding carried out contrary to rules," Xinhua, the official news agency, reported without citing a source.

The report did not say whether those detained were workers or managers.

The swift arrests come as authorities tackle public concern over why the fire took more than four and a half hours to extinguish.

"We feel that the fire rescue measures and methods weren't fast enough, and secondly they weren't vigorous enough," Du Deyuan, a 66-year-old resident who was out when the fire broke out, said.

"People live in high-rises, and then you have this burn all the way from low down to the 28th storey, burnt so the whole building is blazing red. What could the people inside do?"

The blaze started on the scaffolding's 10th storey, and plastic construction sheeting covering the building helped fuel the fire, Zhu Zhixiang, a senior Shanghai fire official, told CCTV state television.

"The building was surrounded by scaffolding covered in a lot of flammable sheeting and plywood boards that caused the fire to spread to a large area in a very short time. The wind was strong and intensified the situation," Zhu said.