Many of the victims were young students who had just been let out of class when a Chinese-made F-7 BJI aircraft slammed into the Milestone School and College.

An AFP photographer at the scene saw fire and rescue officials taking away the injured students on stretchers, while army personnel helped clear the mangled wreckage.

A military statement said 20 people were killed, including the pilot, and 171 others injured when the jet crashed following a mechanical failure.

An 18-year-old student, Shafiur Rahman Shafi, said he heard a huge blast that felt like an earthquake.

"There were two fighter planes... Suddenly one of the two planes crashed here [in the junior playground]," he told AFP. 

"It created a boom, and it felt like a quake. Then it caught fire," he added.

The well-known private school offers education to children from kindergarten through to senior secondary.

Most of the injured were aged between eight and 14, said Mohammad Maruf Islam, joint director of Dhaka's National Burn and Plastic Surgery Institute where many victims were treated.

Grieving relatives of the victims thronged the hospital, while dozens of volunteers lined up to donate blood.

Tofazzal Hossain, 30, broke down in tears on learning that his young cousin had been killed.

"We frantically searched for my cousin in different hospitals," Hossain told AFP.

"He was an eighth grader at the school. Finally, we found his body."

Mohammad Sayedur Rahman, a Health and Family Welfare official, said at least seven bodies remained unidentified.

The military said the pilot, flight lieutenant Towkir Islam, was on a routine training mission when the jet "reportedly encountered a mechanical failure".

"The exact cause remains under investigation," it said in a statement.

The pilot tried to divert the aircraft away from densely populated areas but, "despite his best efforts", crashed into the two-story school building, the military added.

It was Towkir's first solo flight, his uncle Motakabbir told AFP.

"His mother was nervous yet happy and had been eagerly waiting for the completion of her son's first solo flight," said Motakabbir, who uses only one name.

Shuvra Ghosh, the aunt of one of the injured students, said her eight-year-old niece was rescued by a teacher who broke open a window to bring her out.

"We are mentally traumatized," she told AFP.

The interim government of Muhammad Yunus announced a day of national mourning on Tuesday.

Yunus expressed "deep grief and sorrow" over the incident in a post on X. 

"The loss suffered by the Air Force, the students, parents, teachers, and staff of Milestone School and College, as well as others affected by this accident, is irreparable," he said. 

"This is a moment of profound pain for the nation."

The crash was the worst aviation accident in the country in several decades.

The deadliest ever disaster happened in 1984 when a plane flying from Chattogram to Dhaka crashed, killing all 49 on board.

Last month, a commercial aircraft crashed in neighboring India, killing 260 people. 

Indian Prime Minister Narendra Modi said he was "deeply shocked and saddened at the loss of lives" in Dhaka. 

Relations between the neighbors have been strained since protesters in Bangladesh last year ousted leader Sheikh Hasina, an old ally of New Delhi.

"India stands in solidarity with Bangladesh and is ready to extend all possible support and assistance," Modi wrote on X.