At least 27 people, mostly children, were killed after a Bangladeshi fighter jet crashed into a school in the capital Dhaka, a government official said, updating an earlier toll of at least 20 dead.

Most of the victims were schoolchildren who had just been let out of class when the Chinese-made F-7 BJI aircraft slammed into the Milestone School and College on Monday.

"So far, 27 people have died. Among them, 25 are children and one is a pilot," Sayedur Rahman, special assistant to the chief adviser for the Ministry of Health and Family Welfare, said on Tuesday.

"Seventy-eight people are being treated in different hospitals," Rahman added.

The Bangladesh military on Monday had 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-storey school building, the military added.

The interim government of Muhammad Yunus declared that Tuesday would be a day of national mourning.

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."