An "alarming" number of children in Africa, most of them girls, have been used as suicide bombers by Boko Haram in 2017, according to UNICEF.

In the countries fighting Boko Haram in the Lake Chad region - Nigeria, Niger, Cameroon and Chad - 27 children have been used in suicide attacks by the armed group in the first three months of the year, UNICEF said in a report and statement.

There were nine cases in the same period last year, and 30 children used for bombings in all of 2016, it said.

The Boko Haram campaign is now in its eighth year with little sign of ending, having claimed more than 20,000 lives.

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

Its child kidnappings gained global notoriety after the abduction of more than 200 girls from the town of Chibok in Nigeria's northeast in 2014, three years ago on Friday.

"These children are victims, not perpetrators," said Marie-Pierre Poirier, UNICEF's regional director for West and Central Africa.

"Forcing or deceiving them into committing such horrific acts is reprehensible."

One 16-year-old girl from Chad lost her legs after being drugged and forced by Boko Haram to take part in an attempted suicide attack on a crowded market, according to the report.

Though the girl survived, her family initially rejected her "out of fear of stigma".

Children who escape Boko Haram are often held in custody by authorities or ostracised by their communities and families.

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

"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.

READ MORE: UN - 475,000 children at risk in Lake Chad area

On Monday, Nigeria's military released 593 people, including children, after clearing them of having ties with Boko Haram. A UNICEF spokesperson told Reuters news agency that about 370 remain in custody.

"Society's rejection of these children, and their sense of isolation and desperation, could be making them more vulnerable to promises of martyrdom through acceptance of dangerous and deadly missions," the report said.

Children make up 1.3 million of the 2.3 million people displaced by the conflict.