Hundreds of tonnes of highly poisonous cyanide were being stored at the warehouse devastated by two giant explosions in the Chinese port city of Tianjin which killed 112, a senior military officer said Sunday.

The comments by Shi Luze, chief of the general staff of the Beijing military region, were the first official confirmation of the presence of the chemical at the hazardous goods storage facility at the centre of the massive blasts.

The disaster has raised fears of toxic contamination. Residents and victims' families hit out at authorities for what they said was an information blackout, as China suspended or shut down dozens of websites for spreading "rumours".

Nearly 100 people remain missing, including 85 firefighters, though officials cautioned that some of them could be among the 88 corpses so far unidentified.

More than 700 people have been hospitalised as a result of Wednesday's blasts -- which triggered a huge fireball and a blaze that emergency workers have struggled to put out since then, with fresh explosions on Saturday.

State prosecutors said Sunday they have started an investigation to see if dereliction of duty played a role in the disaster, the official Xinhua news agency reported.

In a sign of the potentially horrific nature of some of the injuries, the main state evening news bulletin Sunday showed a hospitalised patient whose head, face and neck were completely encased in thick white bandaging save for openings at the eyes nose and mouth.

Shi, who is a general, told a news conference that cyanide had been identified at two locations in the blast zone. "The volume was about several hundreds of tonnes according to preliminary estimates," he said.

A military team of 217 chemical and nuclear experts was deployed early on, and earlier Chinese reports said 700 tonnes of sodium cyanide were at the site.

Officials have called in experts from producers of the material -- exposure to which can be "rapidly fatal", according to the US Centers for Disease Control -- to help handle it, and the neutralising agent hydrogen peroxide has been used.

Authorities have repeatedly sought to reassure the public, insisting that despite the presence of some pollutants at levels above normal standards, the air in Tianjin remains safe to breathe.

Premier Li Keqiang arrived in the city on Sunday afternoon to direct rescue efforts, a common move after major disasters in the country.

Pictures showed the Communist Party number two within a kilometre (mile) of the blast site, dressed in an ordinary white shirt and not wearing a mask.

Xinhua reported late Saturday that cyanide density in waste water had been 10.9 times standard on the day following the explosions. It has since fallen but was still more than twice the normal limit.

- Testing water -

Environmental campaign group Greenpeace said Sunday it had tested surface water for cyanide at four locations in the city and had not detected high levels of the chemical.

"These results show that local water supplies are not currently severely contaminated with cyanide," it said, but reiterated its call for comprehensive tests on the air and water and for publication of the results.

On Saturday a three-kilometre (two-mile) radius from the site of the blasts was evacuated, state-run media reported. Officials said later the reports were inaccurate but vehicles were turned back at barriers.

On Sunday AFP saw young men, carrying personal belongings, leave FAW Toyota apartments and board a bus waiting to take them to alternative accommodation. Police in masks could be seen at one checkpoint.

Steve Ra, an American who was evacuated by his employer to another area of Tianjin, said he was worried about the potential health effects.

"The main concern is just the air," Ra told AFP. "I'm waiting to go back to get my normal life back. But I don't know what I'll be breathing so that's the biggest concern."

Tianjin residents, relatives of the victims and online commentators have slammed local authorities for a lack of transparency, and tried to storm a news conference on Saturday.

- 'No truth!' -

On Sunday sobbing men confronted security at the hotel where officials have been briefing journalists, with one shouting "Police, I will kill someone!" in what appeared to be a desperate bid to attract attention before being comforted by a policeman.

Another lashed out at reporters attempting to photograph him, saying: "Don't take my photo, it is useless. The news has no truth!"

Outside, residents of a building damaged by the blasts held a protest.

The government has moved to limit criticism of the handling of the aftermath, with a total of 50 websites shut down or suspended for "creating panic by publishing unverified information or letting users spread groundless rumours", according to the Cyberspace Administration of China.

Critical posts on social media have also been blocked, and more than 360 social media accounts have been punished.

One poster on microblogging platform Sina Weibo wrote: "Why is it 'rumours' are flying everywhere every time there is a disaster? Are they really rumours?

"The government is lying... You have lied to the people too much and made yourself untrustworthy."