At least 44 people are now known to have died, and more than 500 injured, following two major explosions in China`s northern city of Tianjin, reports BBC.Twelve firefighters were among the dead, China`s official Xinhua news agency said as it reported a doubling of the death toll.Two blasts happened in a warehouse storing "dangerous and chemical goods" in the port city of the area.A huge fireball lit up the sky, and nearby buildings have collapsed.President Xi Jinping has urged "all-out efforts" to rescue victims and contain the fire, Xinhua reports.The first explosion at about 23:30 local time (15:30 GMT) on Wednesday was followed by another, more powerful blast, seconds later. Shockwaves were felt several kilometres away.The China Earthquake Networks Centre said the magnitude of the first explosion was the equivalent of detonating three tonnes of TNT, while the second was the equivalent of 21 tonnes.Further blasts were subsequently triggered nearby, Xinhua said.State broadcaster China Central Television (CCTV) said a shipment of explosives had detonated but this has not been confirmed.Senior managers of Ruihai Logistics, which owns the warehouse where the explosion happened, are being questioned by authorities, state media reports.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.