YAOUNDE, Cameroon (AP) — Two girl suicide bombers attacked a camp for those displaced by Boko Haram extremist violence and left 11 people dead, authorities in northern Cameroon said Friday.

The dead included the young bombers who detonated their explosives at the camp in Kolofata, said Gov. Midjiyawa Bakari of the Far North region. Several dozen others were wounded, the governor said.

Authorities believed the girls had entered Cameroon the night before from neighboring Nigeria, where Boko Haram has killed more than 20,000 people in its eight-year insurgency that has spread into neighboring countries.

Northern Cameroon has seen a rise in such attacks, with some towns targeted repeatedly. In January 2016, two female suicide bombers attacked a mosque in Kolofata, killing at least 10 people. In September 2015, suicide bombers killed nine people there.

Boko Haram is known for kidnapping girls and using children to carry out suicide bomb attacks. In April, the U.N. children’s agency said at least 117 attacks had been carried out by youth in the Lake Chad basin region since 2014, with nearly 80 percent of the bombs strapped to girls. They are sometimes drugged before missions.

The Islamic extremist group two years ago began attacking in neighboring countries that have supported the Nigerian military’s efforts to counter it. A multinational force is now active in the region.

Nigeria in December declared that Boko Haram had been “crushed” after the military cleared out its strongholds, but attacks have continued.

The U.N. children’s agency on Friday said Cameroon was hosting 96,000 Nigerian registered refugees as of mid-May, but many are now returning. More than 12,000 went back last month.

Many, however, are sleeping in the open just inside the border as “most returnees are still unable to travel onwards to their home villages where security remains uncertain,” UNICEF said.