Chinese soldiers including chemical warfare specialists are racing to clear hazardous materials at the Tianjin blast site ahead of forecast rain which could generate new clouds of toxic gas.

At least 112 people are now known to have died in the massive explosions which ripped through an industrial area of the port city late on Wednesday, hurling huge fireballs into the night sky.

The number of missing rose to 95, most of them fire fighters, state media said, suggesting the toll would rise significantly. More than 720 people remained in hospital and thousands of locals have been evacuated from a two-kilometre exclusion zone around the site of the blasts.

Estimates by Credit Suisse showed the explosions could generate total insurance losses of between $US1 billion and $US1.5 billion.

Insurance companies, including Zurich and Allianz, said they received claims from clients affected by the disaster but could not provide any estimate of the potential losses.

Chinese insurers are also expected to be affected.

Paramilitary policemen in gas masks examine unknown chemicals. ( Reuters: Jason Lee )

Shi Luze, the chief of staff of the People's Liberation Army's Beijing Military Region, told reporters that army teams were working to remove more than 100 tonnes of deadly sodium cyanide, stored at two separate sites.

He said workers were trying to clear the area of chemicals before possible rain showers, which could create toxic gas.

"I can responsibly say that there will be no secondary damage to the people [outside the evacuation zone],"

Officials say the toxins posed no risk to people outside a two-kilometre evacuation zone surrounding the blast area.

Fire crews were criticised for using water to douse flames in the initial fire, which may have contributed to the blasts, given the volatile nature of the chemicals involved.

Greenpeace said tests around the blast site showed water supplies were not severely contaminated with cyanide, but they did not "disprove the presence of other hazardous chemicals in the water".

"Greenpeace reiterates its call for authorities to implement a comprehensive survey of hazardous chemicals currently present in air and water supplies and make public all information," it said.

In an earlier statement, the environmental organisation urged the government to establish a five-kilometre evacuation zone.

An aerial picture shows smoke rising from blown-out shipping containers at the blast site. ( Reuters )

China evacuated residents who had taken refuge in a school near the site of the blasts on Saturday after a change in wind direction prompted fears that toxic chemical particles could be blown inland.

It was not clear from media reports how many people were evacuated, but the order came as a fire broke out again that day at the blast site, a warehouse specially designed to store dangerous chemicals.

Some 6,300 people have been displaced by the blasts. Shockwaves were felt by residents in apartment blocks kilometres away in the city of 15 million people.

China's top prosecutor, the Supreme People's Procuratorate, opened an investigation into the warehouse explosions, state news agency Xinhua reported, and "will look into possible illegal acts, such as abuse of power or dereliction of duty and deal with those acts which may constitute crimes".

About 100 people from a residential area near the blast site protested outside a hotel where a government press briefing was held, angry that dangerous chemicals had been stored near their homes.

Volunteers hand out food and water at a makeshift shelter at the Taida Second Primary School in Tianjin. ( ABC: Huey Fern Tay )

"I'm very worried that these dangerous chemicals will harm my health," said Zhang Yinbao, who works in the chemical industry and whose apartment building is only 800 metres from the blast site.

For a warehouse the size of the one that exploded, that is closer to a residential area than allowed under laws that deal with the storage of dangerous materials, according to state media reports.

"From a legal perspective it's unreasonable that dangerous chemicals would be so close," Ms Zhang said, calling for a thorough investigation and compensation.

Emotional family members of missing fire fighters — 85 of the 95 people missing — marched to district government offices where they scuffled with police before being dispersed.

The mother of a missing firefighter breaks down outside a news conference. ( Reuters: Jason Lee )

Premier Li Keqiang has arrived to offer his condolences.

Industrial accidents are not uncommon in China, following three decades of fast growth. A blast at an auto parts factory killed 75 people a year ago.

ABC/Reuters