CHITTAGONG - 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.Meanwhile, police have abducted the son of a top Bangladeshi war crimes convict, his family said Tuesday, the latest in a series of forced disappearances of opposition leaders' children.

More than 30 plainclothes officers seized Abdullahil Amaan Azmi, formerly a decorated brigadier general in Bangladesh's powerful army, from his home in the capital Dhaka on Monday evening, his family said in a statement.

"The officers cordoned off the whole street before breaking down the door and forcibly entering the family home, blindfolding the caretaker and severely beating him until he fell unconscious," the statement said.

The police had no warrant and gave no reason for arresting Azmi, the family said.

"No official acknowledgement of his arrest has yet been made, without which there are credible fears for a possible extrajudicial abduction," the statement said.

Azmi's father Ghulam Azam, the former head of Bangladesh's largest Islamist party Jamaat-e-Islami (JeI), was sentenced to 90 years in prison in 2013 for alleged war crimes.

The abduction came weeks after similar incidents - involving two other sons of opposition leaders convicted of war crimes - which were blamed on plainclothes policemen in Dhaka. "We don't know anything about these incidents," a national police spokesman told AFP.

Bangladesh's highest court is also set to decide the fate of another top Islamic leader who was sentenced to death in 2014 for war crimes.

If Mir Quasem Ali, who was a top financier of the JeI, loses his final appeal, with hearings expected to start Wednesday, he will be executed within weeks. His son Mir Ahmed Bin Quasem, who was part of his legal defence team, was allegedly abducted earlier in August.

Critics say the abductions are an attempt by police to sow fear and prevent Islamists from staging protests against Ali's imminent execution.

The United Nations on Tuesday urged the government to annul Ali's death sentence and to give him a retrial in compliance with international standards.

Five opposition leaders, including four top Islamists, have been executed for war crimes in the past three years despite rights groups' criticisms that their trials were flawed.