A bomb hidden in a truck exploded in the centre of the Afghan capital Kabul, killing seven people and wounding around 400.

Police chief Abdul Rahman Rahimi said the pre-dawn blast was near a defence ministry compound, but that all of the victims were civilians, including women and children.

Health ministry spokesman Wahidullha Mayar about 400 people were hurt, mostly after being hit by flying glass.

Mr Rahimi said the target of the blast, which blew out windows in nearby homes, was not known.

Police do not allow trucks to enter in Kabul during the daytime, but they can enter from 9pm until 5am.

The blast comes after the United Nations said on Wednesday that a growing number of women and children are getting hurt or killed in Afghanistan's war against the Taliban and other insurgents.

The total number of casualties in the almost 14-year conflict was up one per cent in the first half of this year, compared to the same period last year, a new UN report said.

However, the number of women casualties rose by 23 per cent and children by 13 per cent.