Islamist militant group Boko Haram killed at least 60 people in the Nigerian town Rann a day after it was abandoned by the military, Amnesty International and security sources said.
The attack on Rann, home to a camp housing thousands displaced by the Islamist insurgency, was one of the group’s bloodiest.
It came two weeks after Boko Haram overran Rann, driving out Nigerian soldiers and signalling its re-emergence as a force capable of capturing army bases.
The bloodshed is a challenge for President Muhammadu Buhari as he seeks a second successive four-year term in an election on February 16, having been elected in 2015 partly on a promise to restore security.
A Nigerian army spokesman denied troops left the town and that Boko Haram killed people.
The two attacks have driven some 40,000 people to flee, 30,000 of them into Cameroon, according to aid agencies.
“This attack on civilians already displaced by the bloody conflict may amount to possible war crimes,” Osai Ojigho, Amnesty’s Nigeria director, said in a statement.
“Witnesses told us Nigerian soldiers abandoned their posts the day before the attack, demonstrating failure to protect civilians.”
Security sources told Reuters Nigerian troops returned to Rann with Cameroonian troops from a multinational task force after the first attack on January 14.
After the Cameroonians left, Nigerian soldiers also left because they did not have enough troops, weapons or equipment, the sources said.
The next day, Boko Haram came. The militants set the town ablaze and executed elders and traditional leaders, leaving one alive as a witness, a security source said.
A video of the attack reviewed by Reuters showed people gathering belongings and fleeing, screaming, as the town burned.
Nigerian army soldiers came back to Rann and helped bury some dead after Boko Haram left, Amnesty International and security sources said.
A surge in attacks in the north-east by Boko Haram and its more powerful offshoot Islamic State West Africa Province is throwing preparations for the presidential election into disarray.
Despite violence, Vice President Yemi Osinbajo said “the current rate of return of these IDPs (internally displaced people) and refugees is encouraging and this is due to improving security in many areas once completely out of bounds”.