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.
At least 50 people died and more than 700 were injured, 71 critically, in the explosions on Wednesday evening.
Meanwhile, search teams found a survivor in the debris on Friday.
The survivor was named as 19-year-old firefighter Zhou Ti, state-run news agency Xinhua reported.
Officials said earlier that 17 firefighters were among the dead and 18 others are said to be missing.
Fire crews were already at the site when the explosions took place, having been called to reports of a container fire.
Tianjin's fire department chief Zhou Tian told reporters that more than 1,000 firefighters and 140 fire engines were still battling flames at the scene on Friday.
The Tianjin Port Group Company said dozens of its employees remained unaccounted for, Xinhua said.
The warehouse that exploded in the port area is owned by a company called Ruihai Logistics, which specialises in handling hazardous goods.
The company's website says these goods include compressed gas, flammable liquid and toxic chemicals - some of which become highly explosive when mixed with water.
But city officials said on Friday they still did not know what materials were at the warehouse at the time of the fire, or what the cause of the blasts was. However, they said the site had been sealed off and, as far as they knew, any contaminants were contained in that location.
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.
The BBC's John Sudworth in Tianjin says that once the fire is extinguished, the chemical containers will be moved to a safety zone, away from the heat, so that the contents can be analysed.
But Tianjin's residents remain nervous, he adds, with some posting concerns online about the environmental impact and suspicions that the public may not be hearing the full truth.
Fires are 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.
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The People's Daily, the official newspaper of the Chinese Communist Party, saidthat emergency services were "trying to remove all the 700 tonnes 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.
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.
The two explosions, seconds apart, caused a fireball visible from space and a shockwave that damaged buildings within a 2km radius (1.5 miles). The second of the blasts was the equivalent of 21 tonnes of TNT.
Chinese Premier Li Keqiang has promised "open and transparent information disclosure" on the investigation.