Two huge explosions tore through an industrial area where toxic chemicals and gas were stored in the northeast Chinese port city of Tianjin, killing at least 50 people, including at least a dozen fire fighters, officials and state media said on Thursday.

At least 700 people were injured, more than 71 seriously, the Tianjin government said on its Weibo microblog, and the official Xinhua news agency said two fires were still burning.

Wednesday night's blasts, so large that they were seen by satellites in space, sent shockwaves through apartment blocks kilometres away in the port city of 15 million people. Internet videos showed fireballs shooting into the sky and the US Geological Survey registered the blasts as seismic events.

Vast areas of the port - the 10th largest in the world - were devastated, crumpled shipping containers were thrown around like match sticks, hundreds of new cars were torched and port buildings left as burnt-out shells, Reuters witnesses said.

"I was sleeping when our windows and doors suddenly shook as we heard explosions outside. I first thought it was an earthquake," Guan Xiang, who lives 7 km away from the explosion site, told Reuters by telephone.

Guan, 24, said he saw flames and a mushroom cloud in the sky as he and other residents scrambled to get out of the building.

Tianjin authorities said 12 firefighters were among the 44 killed.

The cause of the blasts was being investigated but Xinhua said several containers caught fire beforehand. 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.

The state-run Beijing News earlier cited Tianjin fire authorities as saying they had lost contact with 36 firefighters. By late afternoon, Xinhua reported 18 were missing, while 66 were among the hundreds of people being treated in nearby hospitals.

Xinhua said 1,000 firefighters and more than 140 fire engines were struggling to contain a blaze in a warehouse that held "dangerous goods".

"The volatility of the goods means the fire is especially unpredictable and dangerous to approach," Xinhua said.

Several fire trucks had been destroyed and nearby firefighters wept as they worked to extinguish flames, the Beijing News reported. - Reuters