LAGOS (AFP, REUTERS) - Gunmen with explosives stormed a Catholic church in southwest Nigeria's Ondo state on Sunday (June 5), killing "many" worshippers and wounding others, the government and police said.

The violence at St Francis Catholic Church in Owo town happened during the morning service in a rare attack in the southwest of Nigeria, where militants and criminal gangs operate in other parts of the country.

No group claimed responsibility for the attack.

The motives and the death toll were not immediately clear, but President Muhammadu Buhari condemned the "heinous killing of worshippers".

Ondo state governor Oluwarotimi Akeredolu said in a statement that it was a "vile and satanic attack".

"We shall commit every available resource to hunt down these assailants and make them pay," he said.

State police spokesman Ibukun Odunlami said the gunmen attacked the church with explosives, leaving an undisclosed number of worshippers dead.

"It's still premature to say exactly how many people were killed. But many worshippers lost their lives while others were injured in the attack," she told AFP.

A doctor at a hospital in Owo, a town in the state in Nigeria's southwest, told Reuters that "several worshippers were brought in dead".

A witness, who gave his name as Abayomi, told AFP that at least 20 worshippers had died in the attack.

"I was passing through the area when I heard a loud explosion and gunshots inside the church," he said.

The man said he saw at least five gunmen on the church premises before he ran away for safety.

Gun and bomb attacks are rare in Ondo state, but Nigeria's military is battling a 12-year-old militant insurgency in the northeast, kidnapping gangs in the northwest and separatist agitation in the southeast.