A succession of explosions Thursday were sparked by a fire at what authorities said were shipping containers containing hazardous material at a warehouse.

The explosions occurred at night and they struck a mostly industrial zone otherwise the death toll could have been much higher.

Firefighters initially responded to a blaze at the warehouse. Many of them were apparently killed by a series of explosions that went off 40 minutes after the fire was reported.

Local officials told a news conference Saturday that the toll included at least 21 firefighters among the more than 1,000 sent to the disaster.

State broadcaster CCTV reported that Zhou Ti, a 19-year-old firefighter, had been pulled from the zone at about dawn Friday and taken to a hospital, where he was treated for face, chest and foot injuries.

There have been questions circulating in Chinese media about whether the firefighters may have sparked the blasts. One report suggested tha water used by first responders may have come into contact with a chemical that explodes on contact with water.

Many of the fire trucks seen at the site hours after the blasts were using a kind of firefighting foam.

Authorities have not said what caused the explosions, saying only that they originated at the warehouse owned by Ruihai International Logistics.

The company's website says it is authorized to handle chemicals ranging from flammable gases and liquids, including compressed natural gas and ethyl acetate, to chemicals that explode on contact with water, including sodium cyanide and calcium carbide.

One day after the blasts, Tianjin's environmental protection chief Wen Wurui told a news conference that there had been no apparent contamination of the air, based on samples taken at five air monitoring stations throughout the city. He also said that all waterways leading from the disaster site to the sea had been sealed to contain any potential water leaks.

av/bw (AP, dpa)