Chinese authorities struggled Friday to extinguish fires and identify dangerous chemicals at a devastated industrial site, two days after giant explosions killed dozens and left residents in fear of being cloaked in a toxic cloud.

Officials in the northern port city of Tianjin, where the blasts killed at least 56 people and injured more than 700, told a news conference they did not yet know what materials were at the hazardous goods storage facility that exploded, or the cause of the blast.

But Chinese media and environment group Greenpeace warned a host of potentially extremely dangerous chemicals may have been stored there.

At least 700 tonnes of sodium cyanide were at the site, along with other dangerous substances, and the poisonous chemical had been detected in nearby drains, the Beijing News initially reported.

But the report was no longer available on the newspaper's website on Friday, giving rise to suspicions that the Chinese government was clamping down on sensitive information relating to the tragedy.

The official Xinhua news agency said a team of 217 nuclear and biochemical materials specialists from the Chinese military had travelled to Tianjin to inspect the site.

Xinhua cited local authorities for the latest rise in the death toll to 56, which it said included 21 firemen.

The agency added that 721 people had been hospitalised, 25 of whom were in critical condition.

Dozens of people were still missing but there was a rare moment to cheer on Friday morning when rescue workers pulled a 19-year-old firefighter from the rubble.

Fear of the unknown

However up to 1,000 firefighters were still struggling to extinguish blazes at the site, with smoke billowing from three areas, adding to uncertainty over whether more chemicals may be leaking.

Some police wore no protective clothing, while others had full-face gas masks, although an environmental expert told an official press conference that toxic gas indicators were within normal ranges and the air "should be safe for residents to breathe".

At a nearby office building, security guard Liu Zongguang, 50, wore a cheap surgical mask.

"I'm wearing this mask because I saw some police wearing them, but I also saw some without masks, I don't really know what to do," Liu said.

"I'm really scared, but I don't even know what to be scared of, the government hasn't said anything, nothing about what we should do to keep our families safe from the chemicals."

Greenpeace warned on Thursday that rain could transfer airborne chemicals into water systems.

The campaign group said it was "critical" that authorities monitored the situation closely and identified what substances were being released into the air.

Tianjin work safety official Gao Huaiyou told reporters that authorities did not know which of the many dangerous substances the company was authorised to store were on the site at the time.

As a trans-shipment facility, items were normally only kept for brief periods and "the types and amount of the dangerous materials are not fixed", he said.

The company's own records were damaged in the blast, he added, and information from its executives was unreliable as it did not match its customs filings.

China's State Council Work Safety Commission said Friday in an emergency notice that it would launch nationwide inspections of businesses involved in dangerous chemicals and explosives, saying "extremely profound" lessons, including a lack of safety awareness, were learned from the Tianjin disaster, Xinhua reported.

The deputy director of the Binhai New Area government, where the blast happened said that 17,000 families, 1,700 industrial enterprises and 675 business had been affected, but did not provide details.

Online commentators expressed fury at authorities, who are regularly accused after disasters of cover-ups.

"Please be honest and say what you know. Don't hide the truth!" one poster urged on social media.

Another said: "Every time when disaster happens, state media always spread heroic acts. In the end, people are touched by these stories. Then nobody asks the cause of the accident to explore the truth."

The People's Daily, the official mouthpiece of China's ruling Communist Party, said that the facility's construction "clearly violated" safety rules.

Under Chinese regulations, warehouses stocking dangerous materials must be at least one kilometre (0.6 miles) away from surrounding public buildings and main roads, it said, but there were two residential compounds and several main roads within that distance.

Two hospitals, a convention centre, several residential compounds and a football pitch were also nearby, it said.

"The warehouse should not have passed the environment assessment under normal circumstances," the paper quoted an unnamed environmental expert as saying.

Residents fearing pollution from proposed or already-built industrial plants in China regularly protest about local authorities being prepared to prioritise profits over safety.

In Tianjin, taxi driver Li Shiwen, 53, expressed familiar sentiments.

"There shouldn't be reckless development of these type of dangerous chemicals so close to where people live," Li said.

"There's always short-term thinking in China, looking for a quick solution," he added. "No one thinks about the fact we have to live here for generations."