An explosion at a chemical fertiliser factory left at least 200 people needing medical treatment after toxic gas spread across large parts of Bangladesh's second city Chittagong, officials said Tuesday.

Police and fire officials said hundreds of residents were also evacuated from their homes near the di-ammonia phosphate (DAP) plant in the port city where a 500-tonne capacity ammonia tank exploded late Monday.

Mesbah Uddin, the district's chief administrator, said that 56 people had been admitted to the Chittagong Medical College Hospital after the gas leak, none of whom was in a life-threatening condition.

‘Eight have been discharged. Forty-eight people are still in the hospital today but they are out of danger,’ he told AFP.

‘Around 150 people were given first aid locally,’ he added.

Firefighters battled through the night to halt the leak and finally declared that the situation was under control in the mid-morning.

‘We used loudspeakers to urge people to evacuate the area. Hundreds did leave but they came back after the situation improved,’ said Uddin.

The local police chief Rafiqul Islam confirmed that hundreds of workers from nearby factories had also been ordered to leave their premises.

Other officials said that strong winds blew the toxic gas across a 10-kilometre radius and there were reports of people being taken ill at the southern city's international airport.

Authorities mobilised a special team of some 60 firefighters after the explosion triggered panic among people living nearby.

‘We saved two other tanks by spraying water,’ Abul Kalam Azad, a senior fire official told AFP, adding that the explosion occurred when a tank was being refilled with ammonia gas.

The government has ordered an investigation into the explosion and a three-member inquiry team was scheduled to visit the fertiliser plant later Tuesday, chief administrator Uddin said.