The death toll from the crash of the Bangladesh Air Force training fighter jet into a school building in Dhaka rose to 27 as more people succumbed to their injuries, authorities said on Tuesday.

The F-7 BGI aircraft, a training fighter jet manufactured in China, experienced a "mechanical fault" moments after takeoff and crashed into the two-storey building of Milestone School and College at Diabari in Dhaka's Uttara area on Monday.

The toll is now 27, and 25 of them are children, Chief Adviser Professor Muhammad Yunus's special adviser, Saidur Rahman, told reporters.

About 170 were injured, with several of them said to be critical.

Wails of despair and pain reverberate at hospitals where patients were treated with burn injuries.

Twenty deaths were reported initially, and seven died of their injuries overnight. The pilot, Flight Lieutenant Mohammad Towkir Islam, was among those killed in the crash.

The government has declared a state day of mourning for Tuesday in memory of those affected by the crash.

A statement from the Chief Advisor's Office on Monday announced that the national flag will be flown at half-mast at all government, semi-government, autonomous institutions and educational institutions across the country.

Special prayers will be organised at all places of religious worship in the country for the injured and the dead.

A high-level investigation committee has been formed by the Bangladesh Air Force to determine the cause of the accident.