An explosion so powerful it was heard several kilometres away killed two people and injured over 30 in China's port city of Ningbo on Sunday while toppling several disused buildings, authorities and residents said, but there was no word as to the cause.

An AFP reporter near the scene of the blast saw heaps of glass from shattered windows in adjacent residential towers, and corrugated-iron storefront security doors bent by the impact.

Local authorities said two people died and two were seriously injured, while state media said more than 30 people were treated in hospitals in Ningbo, just south of Shanghai and one of China's largest ports.

Authorities on Sunday night still had not announced a suspected cause of the explosion, which took place around 9:00am (0100 GMT) in a relatively new residential area in northern Ningbo marked by rows and rows of towering housing blocks.

The city government and state media variously described the site of the blast as a vacant lot or abandoned factory.

But nearby residents said they believed it happened in an expanse of older abandoned apartments and commercial buildings slated for demolition to make way for new housing.

On Sunday night dozens of police guarded the approaches to the area, which were sealed off with police tape, as scores of curious onlookers milled around outside.

"We have no idea what happened, we just have to wait for the leaders to tell us," said an elderly man who gave only his surname He, referring to government officials.

Cars were twisted and mangled by the force of the blast, a plume of grey smoke rose in the sky and debris was scattered for dozens of metres, according to images circulated on China's internet.

"Maybe it was a gas explosion, but I think it was probably explosives. They were going to tear down those buildings anyway," said another local resident who declined to give his name.

"I was more than 10 kilometres (six miles) away picking up my children and I still heard it."

State-run television said Sunday evening that no one else was believed trapped in rubble.

Deadly accidents are common in China, where safety standards are often lax.

- Wasteland of debris -

Aerial images posted by the People's Daily on Twitter showed at least four wrecked buildings around a wasteland of concrete debris.

Helmeted rescuers were seen carrying injured people away from the area, while others stood over a person lying on the ground. Pieces of concrete, wood and glass were shown strewn across a wide area.

The Communist Party's People's Daily said on Twitter that no one lived at the scene but garbage collectors might have been working there.

Police said the cause of the explosion was being investigated.

Some local residents told AFP they were afraid that if problems with the underground gas supply were to blame, there could be more blasts.

Ningbo is one of the country's major international container shipping ports.

China has been rocked by several industrial accidents in recent years.

In 2015 giant blasts killed at least 165 people in the northern port city of Tianjin, causing over $1 billion in damage and sparking widespread anger at a perceived lack of transparency over the accident's causes and its environmental impact.

A government inquiry eventually recommended 123 people be punished.

The official who was mayor at the time of the accident was sentenced to 12 years in prison for graft in September.

Huang Xingguo, 62, had also headed the disaster response committee.