Community leaders however said a higher total of 33 people were killed in the assaults on the remote five villages which took place early on Thursday.
Motorcycle-riding gunmen suspected to be Fulani herders stormed the villages in the predominantly Christian district of Zangon Kataf, shooting residents as they fled their homes, state police spokesman Mohammed Jalinge told AFP.
"The villages were attacked by gunmen on motorcycles in which 21 people were killed and three were injured," Jalinge said.
He said the assailants launched the attacks during a heavy downpour when residents, including local vigilantes, were in bed.
Jalinge said the attacks occurred despite a round-the-clock curfew in the area after violence between the two sides escalated in recent weeks.
The upsurge in killings prompted the state authorities to initiate a truce, but it has failed.
"It is unfortunate the tit-for-tat killings have continued unabated despite efforts by the authorities and civil societies to forge lasting peace among the warring parties," Jalinge said.
He said soldiers and policemen had been deployed to enforce the curfew and prevent further attacks.
Luka Biniyat, a spokesman for the Southern Kaduna socio-cultural union SOKAPU, said "33 people from Atyap ethnic group were killed in the attacks on Apiojyim, Kibori, Apiako, Atakmawe and Magamiya villages where several houses were torched."
Southern Kaduna has for several years been wracked by deadly conflict between Muslim Fulani herders and ethnic Christian farmers over grazing and water rights.
Nigerian President Muhammadu Buhari, a northern Muslim, has blamed the intractable violence on an "evil combination of politically-motivated banditry, revenge killings and mutual violence by criminal gangs acting on ethnic and religious grounds."
Last month, more than 30 people were killed, including 18 wedding guests, in two days of violence in the area.
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