Workers at a construction site in the blast zone evacuate because of safety concerns.[PHOTO/XINHUA]

Seventeen firefighters were among the 50 people killed in the warehouse blasts in the northern Chinese port of Tianjin, authorities said on Thursday.

More than 30 firefighters were still missing, most of them in their 20s or 30s, according to the rescue headquarters in Tianjin.

The initial explosion, at a warehouse storing dangerous chemicals, at about 11:20 pm on Wednesday was followed by a number of smaller blasts. About 700 people were injured, 70 of them seriously.

Lu Yun, head of TEDA Hospital, which has admitted some 150 injured people, said that the injuries were mostly from broken glass or stones.

"The priority is to identify dangerous chemicals and their flammability, then take targeted measures to put out the fire," said a senior official from the Ministry of Public Security's Fire Control Bureau.

He said the bureau had been discussing rescue plans with fire-control experts and would "take emergency and protection measures to cope with sudden blasts and flash fires".

Tianjin Binhai New Area said on its blog that the secondary blast happened just after the first, when the firefighters were conducting search and rescue work.

Chen Shiguang, a senior official from China Armed Police Tianjin Branch, said on Thursday afternoon that 1,500 armed police had been deployed at the site for rescue work, and fires were still burning.

"We have prepared enough food and drinks for the armed police involved. Three hundred of them, wearing masks to protect them from dangerous chemicals, will enter the warehouse to extinguish the fire with sandbags," he said.

According to a statement from the headquarters in Tianjin, the injured firefighters were sent to a nearby hospital for treatment.

Most of them "suffered severe burns and scalding. Some are being treated in the hospital's intensive care unit and are not out of danger", said the statement.

The Tianjin fire department had sent 600 firefighters and 93 fire engines to the site.

"Due to the instability and volatility of dangerous chemicals, it's difficult for us to predict whether there will be sudden blasts or flashovers, and it is dangerous to approach the site," said a firefighter at the scene, who wished to remain anonymous.

The text message.[Photo/Sina Weibo]

Firefighter's messages touch many hearts

Text messages sent by a firefighter to his friend, who is also a firefighter, before he headed to the scene of the explosions in Tianjin early on Thursday have moved many netizens.

Liu Shikang sent the messages to his friend on WeChat at about 2 am.

"If I cannot make it, my dad is your dad. And remember to sweep my mom's tomb," Liu said.

The friend promised to do so and posted the messages on his micro blog, touching the hearts of netizens who forwarded them more than 150,000 times.

Many Internet users left comments under the posts saying: "Please come back safely" and "You firefighters have our highest respect".

Liu returned from the scene safely, said his friend on his micro blog.