At least 145 people were killed in the blast in Chinese port city of Tianjin. PHOTO: REUTERS
The official Xinhua news agency said the dozen formally held include the chairman and senior managers of the firm whose chemical storage facility exploded in the northern city of Tianjin two weeks ago, in the country's highest-profile industrial accident in years.
Read: Blast-ripped industrial zone a hub for Chinese, foreign firms
Separately, the Supreme People's Procuratorate said on its website that prosecutors in the city were probing 11 officials for "abuse of power" and "dereliction of duty" over the blasts, which also injured hundreds of people.
In China, formal arrest normally comes after some time in police detention and sees the case handed to prosecutors, with trial and conviction almost guaranteed.
The 12 arrested include owners of Rui Hai International Logistics who were shown on state television last week, when they were already being held by police, "confessing" to using government connections to obtain safety permits.
The huge explosions left a trail of mangled buildings and burnt out cars in their wake.
Read: China's traffic police on alert after chemical blasts: Xinhua
There are currently 495 troops from the People's Liberation Army chemical defence unit and 66 chemical defence experts assisting in disaster relief operations, Chinese defence ministry spokesman Yang Yujun said on Thursday.
The toll rose to 145 people killed with 28 still missing, according to the Tianjin government's official Sina Weibo account, a Chinese version of Twitter. The post also said 474 are in hospital, including seven in critical condition.
The incident sparked widespread outrage over alleged safety violations by Rui Hai and possible official collusion, and fears of pollutants contaminating the air and water of Tianjin, home to about 15 million people.
Thousands of tonnes of hazardous chemicals were stored at the site, officials have said, including about 700 tonnes of highly poisonous sodium cyanide, a white powder or crystal which can give off lethal hydrogen cyanide gas.
Read: China blast residents evacuated over chemical fears as death toll reaches 104: Xinhua
The warehouse was built within 600 metres (650 yards) of some residential buildings, despite a regulation mandating any hazardous material storage facilities must be at least 1,000 metres away.
Communist authorities and state-run media have sought to pin blame for the disaster on local individuals and officials, rather than systemic factors.
China's powerful State Council, or cabinet, has vowed to conduct a "rigorous" investigation into the cause and has pledged it will publish the findings.
But independent Chinese reporting was quashed in the aftermath according to government censorship notices posted on China Digital Times, a US-based website.
"Websites cannot privately gather information on the accident, and when publishing news cannot add individual interpretation without authorisation," read one notice it cited as sent out by the Cyberspace Administration of China.
Prosecutors said the officials they were investigating came from several government departments including transportation management, customs and work safety, and the president of a state-owned port company in Tianjin.
Industrial accidents are common in China, with corruption thought to be a key factor behind lax enforcement of safety regulations.
Less than two weeks after the Tianjin explosions, an explosion at a chemical plant in east China's Shandong province killed one person and injured nine people.
The head of China's work safety watchdog -- a former vice-mayor of Tianjin -- has been sacked after being put under investigation for corruption, state media said Wednesday.
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