Yaound - Nigerian Islamist insurgent group Boko Haram has killed some 400 civilians since last year in northern Cameroon, with dozens more dying in a "heavy-handed" response by security forces, Amnesty International said in a report published Wednesday.
The report, entitled "Human rights under fire: attacks and violations in Cameroon's struggle with Boko Haram" and published in Yaounde, is based on three research missions in 2015, Amnesty said.
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It said the group has killed at least 380 civilians since January 2014.
Cameroonian security forces have responded with raids on villages and the arrest of more than 1,000 people, including children as young as five, Amnesty said.
At least 25 people have died in custody with more than 130 people missing.
"As Boko Haram has brought its violence to Cameroon, civilians have come increasingly under fire," said Alioune Tine, Amnesty International director for West and Central Africa.
"By killing indiscriminately, destroying civilian property, abducting people and using children as suicide bombers, they have committed war crimes and caused untold fear and suffering to the civilian population."
But Tine said security forces had responded in kind.
"Cameroon's security forces have killed civilians unlawfully or through excessive use of force. People have been arbitrarily arrested, and many held in inhumane conditions which have led to dozens of deaths."
Amnesty quoted one witness as saying Boko Haram fighters shot or slit the throats of at least 30 people in the border town of Amchide in a raid last October.
On April 17, Amnesty added another raid saw more than 100 Boko Haram fighters storm the town of Bia, killing 16 civilians, including two children, as well as torch dozens of dwellings while recent weeks have seen a slew of suicide bombings using girls as young as 13, killing more than 70.
In listing the horrors of Boko Haram's actions, Amnesty criticised the brutality of some security operations undertaken in response.
"The military have used excessive or lethal force. In one cordon-and-search operation at least eight people, including a child, were killed and more than 70 buildings were burnt down in the villages of Magdeme and Double on 27 December 2014."
Many of those arrested "are held in appalling conditions at Maroua prison. Overcrowding, lack of sanitation and inadequate health care led to the death of at least 40 prisoners between March and May 2015 alone," Amnesty said.
Amnesty said at least 130 of more than 200 men and boys arrested in July remained unaccounted for.
"It is unacceptable that nearly nine months after the mass arrest of 200 men and boys, most of their families still do not know whether they are dead or alive," Tine said.
"The scale and depravity of Boko Haram's attacks is appalling and more must be done to protect civilians and bring all those guilty of these crimes to justice. But it is shocking that an army which is supposed to protect civilians from Boko Haram has committed atrocities themselves. Crimes committed on all sides must be immediately and impartially investigated," Amnesty urged.
Boko Haram, behind a six-year-old insurgency aimed at carving out an Islamic state in Nigeria's restive northeast, has made regular forays into northern Cameroon over the past two years.
More than 100 people have been killed in nine suicide attacks since July in the region, leading Yaounde to beef up its military presence with backing from neighbouring Chad.