Massive explosions that rocked the port city of Tianjin were so strong that residents miles away thought they were experiencing an earthquake, witnesses said Thursday as the death toll rose to 50.

A man wearing a mask walks past overturned shipping containers after explosions hit Tianjin. Reuters

Twelve firefighters were among those killed in the twin blasts, which sent massive fireballs into the sky and devastated a warehouse district as well as nearby homes. More than 700 wounded were being treated at hospitals, including 71 in serious condition. No figure was given for those missing.

Chinese authorities did not say what caused the blasts. Local officials said firefighting was suspended on orders of the central government so that a team of chemical experts could assess hazardous materials on site.

As dawn broke over the area, the full extent of the destruction became clear. Buildings had collapsed, homes were littered with shattered glass and smoke was still visible across the scene more than 16 hours after the explosion. Shipping containers were scattered by the force of the blast, leaving them in bent, charred piles.

"It was like what we were told a nuclear bomb would be like"

"We haven't got any formal announcement from local government," local resident Tang Lei told NBC News. "All the information we got so far is from media or friends who are working in the field."

A large explosion followed by an even more powerful blast lit up the night sky over the city — about 75 miles east of Beijing — at about 11:30 p.m. local time Wednesday (11.30 a.m. ET).

"I felt the ground shaking and my door was making a big noise," he said. "I thought it was an earthquake. Then I went to the window and saw a very huge cloud about [over one mile] from my house. Several minutes later, the second explosion happened and we could feel it very strongly. The cloud was as high as a 20-floor building."

Police said an initial blast took place in shipping containers at a warehouse owned by Ruihai Logistics, which says it stores hazardous materials including flammable petrochemicals, sodium cyanide and toluene diisocyanate. The state-backed China Daily newspaper said the company's manager had been detained by police. The company violated packaging standards during a safety inspection two years ago, China's safety bureau said.

The country's National Earthquake Bureau said the first blast had a force equivalent to three tons of TNT, and the second had the equivalent of 21 tons.