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Massive explosions have hit China's northern city of Tianjin, leaving at least 17 people dead and hundreds more injured.
State media said the blasts happened in a warehouse storing "dangerous and chemical goods" in the port area of the city.
Pictures and video on social media showed flames lighting up the sky, and buildings are said to have collapsed.
Hospitals are reported to be overwhelmed with casualties.
President Xi Jinping has urged "all-out efforts" to rescue victims and contain the fire, Xinhua state news agency said.
State broadcaster China Central Television (CCTV) said a shipment of explosives had detonated but this was not confirmed.
The first explosion at about 23:30 (15:30 GMT) on Wednesday was followed by another, more powerful blast, seconds later. Shockwaves were felt several kilometres away.
BBC Chinese Service editor Raymond Li says all indications are that it was an industrial accident.
The China Earthquake Networks Centre said the magnitude of the first explosion was the equivalent of detonating three tons of TNT, while the second was the equivalent of 21 tons of the explosive.
Further blasts were subsequently triggered nearby, Xinhua said.
CCTV said four firefighters were among the dead and that more than 400 people had been injured, at least 32 of them critically.
BBC producer Xinyan Yu, who is in Tianjin, said workers' dormitories were among the buildings destroyed.
John Sudworth, BBC News, BeijingSome of the still photographs being posted online really give a sense of the damage. People are showing internal doors ripped off their hinges and glass is scattered everywhere inside apartments.
The most upsetting images are from the surrounding streets and from hospital waiting areas showing some of the injured - parents carrying injured children.
Hospitals are clearly struggling to cope with this huge influx of casualties.
Hours later, fires were still burning and 100 fire engines were at the scene, reportedCCTV.
One witness, named only as Ms Yang, told local media she was out shopping when "suddenly from behind there was a big fireball and explosion".
"At the time of the explosion the ground was shaking fiercely, nearby cars and buildings were shaking, glass from a few buildings all broke and everyone started to run," she said.
"Now all the residents are gathered in the street."
Another witness, Canadian teacher Monica Andrews, told how she woke in panic after what she thought was an earthquake.
"I... looked out the window and the sky was red. I just watched a second explosion go off and [it was] just pure chaos, everyone leaving their apartment buildings thinking it's an earthquake, cars trying to leave the complex. It was crazy," she told the BBC.
China National Radio said cracks were visible in buildings near the site of the blast.
Several tower blocks near the port area are without power, CCTV added.
Tianjin, home to some 15 million people, is a major port and industrial area to the south-east of the Chinese capital, Beijing,