China has sent a team of military chemical experts to the northern city of Tianjin, after explosions left at least 50 dead, state media report.
The blasts, late on Wednesday night, ripped through an industrial port area, destroying buildings, shipping containers and thousands of new cars.
It is not known what caused the blasts, nor whether chemicals have leaked.
More than 3,500 residents are spending the night in temporary shelters. Hundreds are injured, 71 critically.
State news agency Xinhua reports that rescue workers are "racing against the clock to save the injured and contain fires", 24 hours after the massive blasts at a warehouse in the Binhai New Area.
Many of the wounded had glass or shrapnel cuts, skull injuries or fractures, Wang Siaojie of Teda Hospital said.
More than 200 experts in chemical materials from the Chinese army have arrived in Tianjin. Their first task was to test the air for toxic gases, Xinhua said.
Rescue teams were ordered to wear protective clothing, it added.
The warehouse that exploded is owned by a company called Ruihai Logistics, which handles toxic chemicals including sodium cyanide and toluene diisocyanate, according to reports.
The People's Daily, the official newspaper of the Chinese Communist Party, reports that rescuers "are trying to remove all the 700 tons of sodium cyanide" stored at the site. Hydrogen peroxide had been prepared to detoxify the chemical, the paper says.