The terrorists have for several years been using children to attack crowded markets, mosques and camps for internally displaced people in northeast Nigeria and the broader Lake Chad region.

But the United Nations children's agency said on Tuesday that there has been an appalling increase in the cruel and calculated use of children, especially girls, as so-called "human bombs".

Since the beginning of the year, 83 children have been used to carry out bomb attacks in northeast Nigeria - four times as many attacks as in all of 2016, Unicef said in a statement.

55 of the children used as bombers were girls, most of them under the age of 15, 27 were boys and one was a baby strapped to a girl, UN numbers show.

Since 2014, a total of 125 children have been used as bombers in northeast Nigeria, according to Unicef.

Boko Haram had sometimes, but not always, claimed responsibility for the attacks, the agency added.

It stressed that the children used as "human bombs are, above all, victims, not perpetrators."