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 44 people, including at least a dozen fire fighters, officials and state media said on Thursday.

At least 520 people were injured, more than 60 of them seriously, the Tianjin government said on its Weibo microblog, and the People’s Daily newspaper said four fires were still burning.

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

Vast areas of the port – the 10th largest in the world – were devastated, crumpled shipping con-tainers 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 (4 miles) away from the explo-sion 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 unknown but industrial accidents are not uncommon in China fol-lowing 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, and that another 33 were among the hundreds of people being treated in nearby hospitals.

The official Xinhua news agency said 1,000 firefighters and more than 140 fire engines were struggling to contain a blaze in a warehouse that contained “dangerous goods”.

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“The volatility of the goods means the fire is especially unpredictable and dangerous to ap-proach,” Xinhua said.

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

President Xi Jinping demanded that authorities “make full effort to rescue and treat the in-jured and ensure the safety of people and their property”.

Xi said in a statement carried by official media that those responsible should be “severely han-dled”.

City officials had met recently with companies to discuss tightening safety standards on the handling of dangerous chemicals. The Tianjin Administration of Work Safety posted a notice about the meeting with companies at the port on its website a week ago.