Some bodies had been double-counted when they were being taken to hospital from the blast site following Wednesday's attack, the ministry said on Thursday.

The injury toll was revised to 56 from 67.

Most of the victims were students preparing for university entrance exams in a classroom at the Mawoud Academy in an area of west Kabul where many members of the city's mainly Shi'ite Hazara community live.

Investigators said the bomber entered the centre from a rear entrance and made his way into a classroom where more than 100 students had gathered.

Their funerals were due to take place on Thursday.

There has been no claim of responsibility but the Taliban denied carrying it out.

In a separate incident, a spy agency training facility in Kabul was under attack early on Thursday.

A group of attackers had positioned themselves inside an empty building across the road from the training centre and were shooting at the facility, an agency official said.

Special units had arrived at the scene, Nasrat Rahimi, a spokesman for the Afghan Interior Ministry, said.

Since January, at least 16 attacks have taken place in Kabul, leaving over 240 dead.

The United Nations Assistance Mission in Afghanistan (UNAMA) said last month that 363 children had been killed and 992 others injured in the first six months of the year.

Australian Associated Press