The little girl who survived a building collapse in China in her dead father's arms is a "left-behind" child who had just arrived for a reunion with her migrant worker parents, state media reported Wednesday.

China has hundreds of millions of migrants who have moved from the countryside to its towns and cities in recent decades to find work, their labour fuelling the country's economic boom.

But many remain poorly paid and cannot afford to bring their children -- who would have few rights to school places -- with them, instead leaving them behind to be looked after by relatives, usually grandparents, sometimes only seeing them once a year.

There are estimated to be 61 million so-called "left behind" children and the Zhengzhou Evening News said three-year-old Wu Ningxi was among them.

She lives with her grandfather in the southwestern city of Chongqing, her mother's hometown, and the two only arrived in Wenzhou, on the east coast, on Sunday to visit her parents.

But in the early hours of the next morning the six-storey building in which they lived collapsed, along with three others, killing 22 people including both her parents and her grandfather, it said.

Wu was found alive deep in the debris 14 hours after the disaster, protected by the final embrace of her father, who shielded her from falling rubble with his own body, previous reports said.

The 26-year-old, who came from the central province of Henan and worked in a shoe factory, was found under a thick cement pillar, draped over his daughter.

"When I picked her up from the rubble, she suddenly grabbed my arm tightly. That touched me," the China Daily newspaper cited rescuer Sun Jing as saying. "I comforted her and continued to talk to her."

Wu had only minor injuries, medical staff said according to the Zhengzhou Evening News.

She "apparently had a powerful bond with her grandfather" because she kept asking for him while she was being treated, they said.

The reports did not make clear whether any of her other grandparents were still alive and said a charitable organisation would fund her living expenses temporarily.

Police have detained four owners of the caved-in houses -- built by the villagers themselves -- and the cause was being investigated, the Xinhua news agency reported Wednesday.

Recent heavy rainfall combined with the poor quality of construction and age of the buildings, were probable contributing factors, earlier media reports said.

Neighbouring buildings constructed in the 1970s were being demolished to prevent further collapses, they said.

The apartments had been packed with migrant worker tenants, who under Chinese law face restrictions on buying homes in the areas where they work.