Xinhua news agency reported that the local residents were evacuated after highly-irritant chlorine gas leaked from the Tianyuan Chemical Industry Plant on Thursday evening in a densely populated neighbourhood of Chongqing city.

Local authorities moved to evacuate all residents within one-kilometre radius of the factory, waking many up from their slumber.

But by Friday evening, three of the factory's huge chlorine tanks exploded before the emergency crews could control the leak, forcing more evacuations.

"Since the explosions of chorine gas, so far 150,000 people have been evacuated," Xinhua said.

The news agency later listed seven senior staff and a worker among nine people dead or missing but said three people who were injured were out of danger.

Toxic fumes

"Initial evaluation shows the explosions were caused by antiquated equipment and improper operation," Miao Guangkui, director of the rescue team said.

Firefighters estimated at least seven of the plant's liquefied chlorine gas furnaces, with a load of six tons of chlorine gas each, were letting off toxic fumes.

Inhalation of 2.5 milligrams of chlorine gas is enough to cause death.

In December, a massive blast at another facility in the city released a cloud of toxic gas, killing 243 people.