Rescuers raced late Monday to try to save victims of a huge China landslide which left 85 people missing after signs of life were detected under a sea of mud, state media said.

The landslide caused by the collapse of a vast soil dumpsite buried 33 factory and residential buildings in the southern city of Shenzhen, China's second industrial disaster in four months.

Rescuers using cutting gear were close Monday evening to reaching the first floor of a buried office building but were "racing against time", the official Xinhua news agency reported without giving details of the life signs.

"The rescue is extremely difficult with mud and silt filling up the excavation," it quoted firefighter Cui Bo as saying.

Witnesses described a mass of red earth and mud racing late Sunday morning towards an industrial park in the city in "huge waves" before burying or crushing homes and factories, twisting some of them into grotesque shapes.

"I saw the houses collapse, all the factories got buried," Liu Youqiang, 45, told AFP.

He was heading home for a meal when disaster struck: "I could only step back and dared not step forward."

Drone footage showed waves of mud had swept through and over buildings and tossed aside trucks like discarded toys.

One weeping migrant worker told how he lost contact with 16 friends and family members after his home was buried.

The landslide covers an area of 380,000 square metres -- about 60 football fields -- and in many areas is more than 10 metres thick, said Liu Qingsheng, vice mayor of the city bordering Hong Kong.

"It is the first time in China that we have seen a landslide on this scale," said Liu Guonan of the China Academy of Railway Sciences.

"The soil on the slope is very high in water content so it's hard to even walk across it -- people's feet sink into it," he added.

Xinhua said 85 people were missing, adding authorities had revised down the previous estimate of 91. Almost 3,000 people were involved in the rescue.

The landslide was caused by the improper storage of waste soil from construction sites, according to the newspaper of the Ministry of Land and Resources.

The soil was allegedly illegally stored in heaps 100 meters (330 feet) high at an old quarry site and turned to mud during rain Sunday morning, the state-run Global Times reported.

Industrial accidents are common in China, with safety regulations often overlooked due to corruption. An explosion in August in the port city of Tianjin that killed nearly 200 people was blamed on improperly stored chemicals.

In Shenzhen about 900 people were evacuated safely and 16 people were taken to hospital, Xinhua said, adding only seven people have been rescued so far from the debris.

- Mounds of dirt -

Mounds of dirt three stories high stood at the edge of the clean-up area Monday while dozens of earthmovers worked in the distance.

"I don't think there will be a chance (to save anybody)," a woman who identified herself as Qin told AFP.

She was one of a group of local volunteers who wanted to help with the rescue but were turned away by authorities.

Photos showed victims wrapped in green blankets sleeping on mattresses and eating instant noodles.

He Weiming, a migrant worker at a temporary shelter, said he had lost contact with 16 friends and family members, including his parents, wife and two children, and had made dozens of phone calls to try to find them. All went unanswered.

"When my brother and I drove out in the morning to go collect garbage, our home was still fine, but when we came back... the house had been buried in mud -- you couldn't even see the roof of our four-metre-high sheet metal house," he told an online news site run by Tencent -- weeping and flipping through photos of his children on his cellphone.

"There are many other homes around mine -- I don't know if others escaped."

A female factory worker surnamed Wang barely made it.

"Seeing the mud approaching us like sea waves, I started running at once and dared not look back. I felt I would have been engulfed in it if I were just one second late," she told Xinhua, adding that a worker who tried to retrieve his motorcycle was buried.

The slide ruptured a natural gas pipeline and triggered an explosion Sunday heard about four kilometres (2.5 miles) away, Xinhua said.