The site should have been closed in February, but workers said mud and waste had continued to be dumped there, a news portal run by authorities in the southern city of Shenzhen said.
Premier Li Keqiang ordered an investigation into Sunday’s landslide in the city, just across the border from Hong Kong.
The mudslide smashed into multi-storey buildings at the Hengtaiyu industrial park in the Guangming New District, toppling them within seconds in collisions that sent rivers of earth skyward.
“The mud had been building up for a few years," said Han Bin, who lives by the site and witnessed the wall of mud sweep towards the buildings on Sunday.
“We didn’t realise this could happen."
The frequency of industrial accidents has raised questions about safety standards after three decades of breakneck growth in the world’s second-largest economy. Just four months ago, more than 160 people were killed in big chemical blasts in the northern port city of Tianjin.
State television showed devastation in Shenzhen, with bits of broken buildings sticking up from heaps of mud stretching out over the industrial park.
More than a year ago, a government-run newspaper warned Shenzhen would run out of space to dump waste from a building frenzy.
Besides new buildings, a network of subway lines is being built in Shenzhen, and large volumes of earth are being excavated and dumped at waste sites.
“Shenzhen has 12 waste sites and they can only hold out until next year," the official Shenzhen Evening Post, published by the city government, said in October last year.
Once a sleepy fishing village on the Communist side of a Cold War frontier, Shenzhen was chosen by Beijing three decades ago to help pioneer landmark economic reforms, and it has boomed ever since.
The mudslide at the business park had covered an area of more than 380,000 square metres (94 acres) and was 10 metres (11 yards) deep in parts, Shenzhen vice mayor Liu Qingsheng told reporters, according to Xinhua.
Almost 3,000 rescuers were at the scene, Xinhua said, with sniffer dogs and drones. Rescuers were focusing on several areas where sensors had detected signs of life, it added.
Unstable waste mountain
The ministry of land resources said the accumulation of a large amount of waste meant that mud was too steep, “causing instability and collapse, resulting in the collapse of buildings".
Media said no foreign companies were believed to have been affected.
A nearby section of China’s major West-East natural gas pipeline exploded, state television said, though it was not clear if this had any impact on the landslide.
Xinhua said the pipeline was owned by PetroChina, China’s top oil and gas producer, that the 400-metre-long ruptured pipe “has been emptied" and a temporary pipe will be built.
PetroChina wrote on its microblog the pipeline blast had hit at least one industrial user, a Hong Kong power plant operated by Castle Peak Power Co Ltd, a company majority owned by a subsidiary of CLP Holdings, that had switched to coal for power generation.
Fourteen factories, 13 low-rise buildings and three dormitories were among the buildings flattened.
Xinhua said the government revised the number of missing to 85 from 91. It did not say why.
Xinhua said 14 people had been rescued and more than 900 people had been evacuated from the site by Sunday evening. State television said the 85 missing included 59 men and 32 women. Reuters