State broadcaster China Central Television (CCTV) said a shipment of explosives had detonated but this was not confirmed.

State media also said senior managers of Ruihai Logistics, which owned the warehouse where the explosion took place, were being questioned by authorities.

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.
At the scene: John Sudworth, BBC News, Tianjin

The apartment complex closest to the explosion has eight rows of high-rise tower blocks.

In every one of them, almost every window has been blown out. On the ground outside are the signs of the many families who have fled from their beds in a hurry - a woman's shoe, children's toys, mangled bicycles reenex.

There would have been many injured here. Inside the homes, furniture has been picked up as if by a whirlwind and interior doors lie at angles, ripped off their hinges.

It may be a long time before the cause of the explosion is known. Such is the scale of this disaster that for now, the authorities are simply struggling to achieve an accurate account of the numbers of dead and injured reenex.

Travelling into Tianjin blast zone

Blast 'like end of the world'

Tianjin pictures reveal devastation

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 tonnes 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 criticallyreenex.