National mourning turned to anger in Bangladesh on Tuesday as the death toll from a fighter jet crash into a school in Dhaka jumped to 31, sparking protests by hundreds of students against the interim government in a country gripped by instability.

At least 25 of the dead were children, many under the age of 12, who were about to return home on Monday when the Chinese-manufactured F-7 BGI Bangladesh Air Force jet ploughed into Milestone School and College and burst into flames, trapping pupils in the fire and building debris.

Their fellow students and others from nearby schools protested as two government officials visited the crash site, demanding justice and shouting, "Why did our brothers die? We demand answers!"

Elsewhere in the capital, hundreds of protesting students, some of them waving sticks, broke through the main gate of the federal government secretariat, demanding the resignation of the education adviser, local TV footage showed. Police baton charged them and forced them out.

Rescue workers continued to scour the charred buildings for debris on Tuesday as distressed residents of the area looked on. Some parents were inconsolable.

"I took her to school yesterday morning like every day. I had no idea it would be the last time I would be seeing her," said Abul Hossain, breaking down as he spoke about his nine-year-old daughter, Nusrat Jahan Anika, killed in the crash. She was buried on Monday night.

Rubina Akter said her son Raiyan Toufiq had a miraculous escape after his shirt caught fire when he was on a staircase.

"He sprinted to the ground floor and jumped on the grass to douse it," she said. "He tore his shirt and vest inside which saved him from severe burns."

The jet had taken off from a nearby air base on a routine training mission, the military said, adding the plane experienced a mechanical failure and the pilot was among those killed. Although he tried to divert the aircraft away from populated areas, the jet crashed into the campus.

On Tuesday, the military said in a statement that 31 people had died and 165 had been admitted to hospitals in the city. The health ministry later said 70 were still under treatment.

The government announced a day of mourning, with flags at half-mast and special prayers at all places of worship.

STUDENT DEMANDS

The protesting students called for those killed and injured to be named, for air force compensation to the families of those killed, the decommissioning of what they said were old and risky jets, and a changing of air force training procedures.

A statement from the press office of Muhammad Yunus, the country's interim administrator, said that the government, the military, school and hospital authorities were working together to publish a list of victims.

It also said the air force will be instructed to not operate training aircraft in populated areas.

The F-7  BGI is the final and most advanced variant in China's Chengdu J-7/F-7 aircraft family, according to Jane's Information Group. Bangladesh signed a contract for 16 aircraft in 2011 and deliveries were completed by 2013.

The Chengdu F-7 is the licence-built version of the Soviet era MiG-21.

The incident comes as neighbour India is still grappling with the world's worst aviation disaster in a decade after an Air India plane crashed into a medical college hostel in Ahmedabad last month, killing 241 of the 242 people on board and 19 on the ground.

Bangladesh has faced months of political uncertainty after then Prime Minister Sheikh Hasina was forced to flee the country last August following weeks of deadly student protests.

The interim government of Nobel laureate Yunus has promised to hold elections next year amid mounting demands from political parties to advance them.