UNICEF is reporting a four-fold increase in use of children as "human bombs" by the radical Boko Haram group in northeastern Nigeria already this year compared to all of 2016.

UNICEF spokeswoman Marixie Mercado says she has no explanation for the increase to 83 such "human bomb" children, two-thirds of them girls, compared to 19 total children last year.

She said Boko Haram does not always claim responsibility for such attacks typically against civilian targets, but no other groups are known to use the tactic.

A side-effect is that "many children who have managed to get away from captivity face rejection when they try to reintegrate into their communities."

The U.N. estimates the insurgency has displaced 1.7 million people and left an estimated 20,000 people dead since 2009.