Forces from Chad and Niger reportedly uncovered an execution site containing at least 70 bodies with slit throats after entering Damasak, a town in northeastern Nigeria controlled for months by Boko Haram.
The bodies, some of which had been decapitated, were found spread out over a dry riverbed under a concrete bridge, a witness told Reuters on Saturday. The killings may have occurred weeks or months ago, as the corpses had been partially mummified by the desert air, the witness said.
A trail of blackened blood smeared on the side of the bridge suggested the people had been thrown off after being killed, the witness added.
The number of victims, all unidentified, could be much higher. Chadian army Col. Azem Bermandoa Agouna told AFP that soldiers saw 100 bodies under the bridge. The Reuters witness also said the smell of decomposing flesh indicated more bodies could be strewn throughout the town.
Most of the town's resident fled long before allied forced recaptured the town last week, but around 50 people who were too frail or sick to move remained behind, Reuters reported. One witness who escaped the insurgents said he saw the militants chasing residents into nearby bush country.
"People were in town when they (Boko Haram) attacked, they fired at us, we ran away to the bushes but they continued to fire and chased some people to kill them," Mbodou Moussa told Reuters.
The Chadian army spokesman said their forces called for Nigerian soldiers to take over the occupation of the town, which lies close to the border of both countries, but Chadian troops would stay there until reinforcements arrived.
Boko Haram reportedly overran the town in Borno state months ago as part of its campaign to capture the northeast of Nigeria and establish an Islamic caliphate there.
The militant group has killed and kidnapped thousands of residents from local towns and villages in that time, and caused many more to flee into neighboring countries.
The latest violence comes as Nigeria gears up to hold its presidential election on March 28, which was already postponed once in February due to fears the rebels would act on previous threats of mass violence as locals headed to polling booths.
Forces from Nigeria, Niger, Chad, and Cameroon have banded together in recent months to fight the insurgency, and Nigerian President Goodluck Jonathan stated this week that he believes the forces will flush out Boko Haram from the territories it controls within a month.