Authorities had lost contact with 36 firefighters on the scene, the official Beijing News newspaper reported, citing the Tianjin fire department.
The force of the explosions unnerved residents across much of the city of 15 million people, with some posting videos on the Internet that showed giant fireballs shooting into the sky and shockwaves buffeting apartment blocks and cars.
Fires were still burning after dawn, with photographs on Chinese news websites showing what appeared to be several destroyed buildings as well as torched cars at a multi-story car park inside a logistics base at Tianjin Port. The port, one of the busiest in China, was operating normally, a port official said.
President Xi Jinping demanded that authorities quickly extinguish the fires and "make full effort to rescue and treat the injured and ensure the safety of people and their property", China Central Television (CCTV) said on its official microblog.
CCTV said the blasts erupted in a shipment of explosives at around 11:30 p.m. local time (1130 EDT), triggering a shockwave that was felt kilometers (miles) away. The second blast came roughly 30 seconds after the first, state media said.
Tianjin is home to around 15 million people, making it one of the biggest cities in China.
Industrial accidents are not uncommon in China following three decades of breakneck economic growth. A blast at an auto parts factory in eastern China killed 75 people a year ago when a room filled with metal dust exploded.
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05:04
Baku-APA. A huge explosion rocked the northern Chinese port city of Tianjin on Wednesday, state media reported, with the cause and number of casualties as yet unknown, APA reports quoting AFP.
The official Xinhua news agency said the blast happened at around 11:30 pm (1630 GMT), when a deafening bang was heard as plumes of flame lit up the sky, scattering dust dozens of metres into the air.
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