The attackers tied up agricultural labourers working in rice fields and slit their throats near Maiduguri, the capital of Borno state, reports say.
This is one of the worst attacks in recent months in a region where the Boko Haram and the Islamic State West Africa insurgent groups are active.
No-one has yet claimed responsibility.
“I condemn the killing of our hard-working farmers by terrorists in Borno state. The entire country is hurt by these senseless killings. My thoughts are with their families in this time of grief. May their souls rest in peace,” said President Muhammadu Buhari.
Mr Buhari also described “the terrorist killings as insane”, according to his spokesman Garba Shehu.
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