Explosions at a warehouse for dangerous materials in the northeastern Chinese port of Tianjin have killed at least 44 people, including at least a dozen firefighters, and injured nearly 400 and sent shockwaves through the city.

Chinese state news said the blasts started late on Wednesday after a container of "hazardous material" exploded in a warehouse around midnight local time.

State news agency Xinhua reported that at least 44 people had died and 32 of the injured were in critical condition in hospital after the explosions. At least 36 firefighters were initially reported missing by the state news agency.

The blasts knocked doors off buildings in the area and shattered windows up to several kilometers away.

There were no indications of what caused the blasts, and no immediate signs of any large release of toxic chemicals into the air.

Beijing News reported on its website that there was some unidentified yellow foam flowing at the site.

Police in Tianjin said an initial blast occurred at shipping containers in a warehouse for hazardous materials owned by Ruihai Logistics, a company that says it is properly approved to handle hazardous materials.

Al Jazeera's Adrian Brown, reporting from Tianjin early on Thursday, said: "Close to the disaster zone, dazed people are wandering about the streets, many carrying what possessions they could grab before fleeing their homes.

"Others are sitting at roadsides, many clearly in shock. Those who can get out are fleeing.

"In the distance, smoke is still billowing from the scene of the multiple blasts. Scores of nearby buildings have had their windows punched out.

"The streets are littered with broken glass and stones. The air is acrid. That no one knows what they are breathing is adding to the anxiety here."

State media said senior management of the company had been detained by authorities, and that President Xi Jinping has demanded severe punishment for anyone found responsible for the explosions.

The official Xinhua news agency said an initial explosion sparked other blasts at nearby businesses.

The National Earthquake Bureau reported two major blasts before midnight, the first was the equivalent of three tons of TNT, and the second one was the equivalent of 21 tons.

The explosions occurred in a mostly industrial zone, with some apartment buildings in the vicinity.

Buildings of a half-dozen other logistics companies were destroyed in the blasts, and more than 1,000 new Renault cars were left charred in a nearby parking lot, Beijing News said.

Photos taken by bystanders and circulating on microblogs show a huge fireball high in the sky, with a mushroom cloud.

Tall plumes of smoke

Other photos on state media outlets showed a sea of fire that painted the night sky bright orange, with tall plumes of smoke.

In one neighborhood about 10km to 20km from the blast site, some residents were sleeping on the street wearing gas masks, although there was no perceptible problem with the air apart from massive clouds of smoke seen in the distance.

At the nearby Taida Hospital as dawn broke, military medical tents were set up.

Photos circulating online showed patients in bandages and with cuts.

State broadcaster CCTV said six battalions of firefighters had brought the ensuing fire under control, although it was still burning in the early hours of Thursday.

Ruihai Logistics says on its website that it was established in 2011 and is an approved company for handling hazardous materials. It says it handles one million tons of cargo annually.

Tianjin, with a population of about 15 million, is about 120km east of Beijing on the Bohai Sea and is one of the country's major ports.

It is also one of China's more modern cities and is connected to the capital by a high speed rail line.

PHOTO CAPTION

Photos on state media outlets showed a sea of fire that painted Wednesday's night sky bright orange in Tianjin [Reuters]

Aljazeera