Gunmen who attacked a Catholic church on Sunday with explosives in southwest Nigeria killed at least 21 worshippers, including some children, local officials said.

The violence in St Francis Catholic Church in Owo town during a Sunday service prompted international condemnation after a rare attack in Nigeria's usually safer southwestern region.

Richard Olatunde, spokesman for the Ondo State governor's office, told AFP that dynamite exploded inside the church before gunmen opened fire.

"They did not even enter the church, they were shooting through the windows," he said, confirming a death toll of 21 people.

National Emergency Management Agency local representative Olanrewaju Kadiri said 22 people were killed, including several children.

No group immediately claimed responsibility for the attack and the motives were not clear.

One witness told AFP he saw at least five gunmen on the church premises during the attack.

President Muhammadu Buhari condemned "heinous killing of worshippers" while Pope Francis offered prays for the victims.

The UN Special Representative for West Africa and the Sahel, Mahamat Saleh Annadif, condemned what he called a "barbaric terrorist attack."

Nigeria is still fighting a 12-year-long jihadist insurgency in its northeast and heavily armed criminal gangs often carry out raids and mass kidnappings in the northwest.

But large attacks in the country's southwest are relatively rare although kidnappings have become increasingly common.

Boko Haram jihadists in the northeast have targeted churches in the past. The conflict has killed 40,000 and displaced two million more.