China has ordered the evacuation of residents within a 2 miles radius of the Tianjin blast site over fears of chemical contamination.
The evacuations came as police confirmed the highly toxic chemical sodium cyanide was found near the site.
A man was found alive 200ft from the blast core, Xinhua news agency said.
Eighty-five people are now known to have died and hundreds were hurt in the giant blasts in the north-eastern Chinese port on August 12.
People sheltering at a school used as a safe haven since the disaster have been asked to leave wearing masks and long trousers, reports say.
Anti-chemical warfare troops have entered the site.
The People’s Daily newspaper tweeted that they had been sent to handle highly toxic sodium cyanide which had been found there.
The discovery was confirmed by police “roughly east of the blast site” in an industrial zone, state-run Beijing News said.
Officials had until then only confirmed the presence of calcium carbide, potassium nitrate and sodium nitrate. Calcium carbide reacts with water to create the highly explosive acetylene.
Officials have so far insisted that air and water quality levels are safe.
Some fires have continued to smolder and at least one reignited on Saturday. Xinhua said several cars at the site had “exploded again”.
Of the 721 people injured, 25 are in critical condition and 33 are serious.
At least 21 firefighters are among the dead and an unknown number remain missing.
Dozens of relatives of the missing tried to disrupt a news conference given by officials on August 15, demanding to know the fate of their loved ones.
The operators of the Tianjin site have been accused of violating safety rules.
China’s government has ordered officials to make nationwide checks on dangerous chemicals and explosives and to “crack down unwaveringly on illegal activities to ensure safety”.