An "alarming" number of children, most of them girls, have been used by Boko Haram as suicide bombers in the first months of 2017, UNICEF said Wednesday.

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

Experts said the number of children used in suicide attacks by Boko Haram surged to 27 in the first quarter of this year, compared to nine over the same period in 2016.

Since 2014, 117 children -- the "vast majority" of them girls -- have been used to carry out attacks in public places across Nigeria, Chad, Niger and Cameroon, said the report by the United Nations children's agency UNICEF.

Four children were used to carry out bomb attacks in 2014, 56 in 2015, 30 in 2016 and 27 in the first three months of 2017, reported UNICEF.

"The increase reflects an alarming tactic by the insurgents," said the report.

"This is the worst possible use of children in conflict," said Marie-Pierre Poirier, UNICEF's regional director for West and Central Africa.

In a separate statement, UNICEF said it was concerned that children were being held by the Nigerian military for alleged association with Boko Haram militants.

"They are held in military barracks, separated from their parents, without medical follow-up, without psychological support, without education, under conditions and for durations that are unknown", said Patrick Rose, a UNICEF regional coordinator.