Boko Haram extremists have killed more than 380 people in the Lake Chad region since April, a major resurgence of attacks that has doubled the casualties compared with the previous five months, Amnesty International said.
The spike in attacks by the Nigeria-based Islamic militants is a result of increased use of suicide bombers, often women and girls, who target highly populated areas in Cameroon's Far North region and Nigeria's Borno and Adamawa states, the rights group said.
Cameroon has had at least one suicide attack per week, it added.
"Boko Haram is once again committing war crimes on a huge scale, exemplified by the depravity of forcing young girls to carry explosives with the sole intention of killing as many people as they possibly can," said Alioune Tine, Amnesty's director for west and central Africa.
"This wave of shocking Boko Haram violence, propelled by a sharp rise in suicide bombings, highlights the urgent need for protection and assistance for millions of civilians in the Lake Chad region."
Nigeria's military has pushed more Boko Haram fighters from the Sambisa Forest in Nigeria to the Mandara Mountains in Cameroon, a possible explanation for the increased attacks in Cameroon, Amnesty said.
Extremists have been crossing into and attacking towns in neighbouring countries, including Cameroon and Niger, that contribute to a regional military force trying to eliminate the insurgency.
Nigeria and Cameroon's governments should take swift action to protect civilians in need of humanitarian assistance, said Amnesty. Boko Haram's increased attacks have made it difficult to carry out humanitarian operations, it said.
Boko Haram has killed more than 20,000 people during its eight-year insurgency. The violence has displaced at least 2.3 million in the region, Amnesty said, adding that seven million are facing serious food shortages.
"The international community should also rapidly scale up its commitment to provide life-saving humanitarian assistance to the millions in the region who need it," said Mr Tine.
AP