DAKAR, Senegal — A girl perhaps no more than 10 years old detonated powerful explosives concealed under her veil at a crowded northern Nigeria market on Saturday, killing as many as 20 people and wounding many more.

The blast inflicted devastating damage on shoppers at the Monday Market in Maiduguri, the shopping hub in a city that is at the heart of the Boko Haram insurgency. The explosion, witnessed by dozens of people, represented a new tactic in the Islamists’ campaign with their decision to use perhaps their youngest-ever suicide bomber.

The terrorist group has increasingly employed women as suicide bombers, even as it has stepped up its abductions of girls across northeast Nigeria, including the kidnapping of more than 200 in the town of Chibok last April. Late last year, two women hit the Monday Market in suicide attacks, killing dozens, and in one week last summer four women carried out bombings in northern Nigeria’s biggest city, Kano.

But the use of a child to kill — witnesses, police officials, a top hospital official in Maiduguri and local vigilantes all agreed that the bomber was very young — may be unprecedented in the insurgency.