Fires are still burning at the site of two massive explosions in the Chinese city of Tianjin, some 36 hours after the blasts.

A team of military chemical experts is testing for toxic gases at the scene and rescuers have been ordered to wear protective clothing, state media said.

Thousands spent the night in emergency shelters in Tianjin

At least 50 people died and hundreds were injured, 71 critically, in the explosions on Wednesday evening.

Meanwhile, search teams found a survivor in the debris on Friday.

The survivor, a firefighter, had been pulled from the ruins of a warehouse, officials said.

The warehouse that exploded in the port area is owned by a company called Ruihai Logistics, which reportedly handles toxic chemicals including sodium cyanide.

City officials said on Friday they still did not know what materials were at the warehouse, or what the cause of the blasts was.

Gao Huaiyou, deputy director of Tianjin's work safety watchdog, told reporters that there were major discrepancies between the accounts of company managers and customs officials. He said damage to the company's office also made it difficult to identify the chemicals.

But officials said the site had been sealed off and, as far as they knew, any contaminants were contained in that location.

Fires were still sending plumes of smoke over the destroyed buildings, burned out vehicles and crumpled shipping containers that bore the brunt of the explosions.

More than 3,500 residents made homeless by the blasts spent the night in temporary shelters.

The People's Daily, the official newspaper of the Chinese Communist Party, said that emergency services were "trying to remove all the 700 tons of sodium cyanide" stored at the site.

The paper said that hydrogen peroxide had been prepared to detoxify the chemical.

It added that the facility's construction "clearly violated" safety rules.

Rows upon rows of new vehicles were destroyed by the explosions

The head of Tianjin's environmental protection bureau, Wen Wurui, said pollution levels were being monitored.

"It would be harmful if you breathe in [this toxic air] for a long time," he said.

"But at present, it has not exceeded [the standard] too much based on our monitoring."

Water discharge points to the sea had been closed, he said.

At least 17 firefighters are among the dead, state-run news agency Xinhua said. Fire crews were already at the scene when the explosions took place, having been called to reports of a container fire.

The Tianjin Port Group Company said dozens of its employees were unaccounted for, Xinhua added.

The two explosions, seconds apart, caused a fireball visible from space and a shockwave that smashed windows several kilometres away.

Chinese Premier Li Keqiang has promised "open and transparent information disclosure" on the investigation.