Fighters from Daesh West Africa Province drove into remote Felo village in Gubio district in the early afternoon, shooting fleeing residents and running them over with their vehicles.
"Boko Haram terrorists" raid Blabrine military base in troubled Diffa region leaving another 10 soldiers wounded, defence ministry says in statement.
An autopsy carried out on four of the dead prisoners revealed traces of a lethal substance that had caused heart attacks in some of the victims and severe asphyxiation in the others, Chad's chief prosecutor said.
Officials say the death toll could climb after militants, who arrived on motorbikes and in cars, attacked a military base in the Gao region.
Nigeria's military said attacks by militants from Boko Haram and a splinter group affiliated with Daesh had "necessitated the contiguous nations of the Lake Chad basin to jointly launch this massive onslaught."
"It's the first time we have lost so many men," President Idriss Deby Itno says, after visiting attack site in Lac province, which borders Niger and Nigeria.
Insurgents fired rocket-propelled grenades at a lorry carrying troops as it travelled near Gorgi village in Borno state, two military officers said without giving their names.
Local sources said a "major" attack had killed soldiers who were hospitalised in Diffa, but they did not give figures.
A stampede was triggered during the distribution of food and money, $13.75 per person, in Diffa, the main town of a southwest region of that name that abuts Nigeria and Chad.
Militants targeted the village of Auno on a key highway linking to regional capital Maiduguri, officials say, adding many women and children were abducted in the raid.
Despite boycott and separatists violence, President Paul Biya –– one of world's oldest and longest-serving leaders, who has held a tight grip on power for 37 of his 86 years – is billed to win.
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