(AFP) - Gunmen with explosives stormed a Catholic church and opened fire in southwest Nigeria on Sunday, killing "many" worshippers and wounding others, the government and police said.

The violence at St. Francis Catholic Church in Owo town in Ondo State erupted during the morning service in a rare attack in the southwest of Nigeria, where jihadists and criminal gangs operate in other regions.

Pope Francis said in a statement he had learned of the "death of dozens of faithful", many children, during the celebration of the Christian holiday of the Pentecost.

"While the details of the incident are being clarified, Pope Francis prays for the victims and for the country," he said.

No group claimed responsibility for the attack.

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

State police spokeswoman Ibukun Odunlami said the gunmen also attacked the church with explosives, leaving an unknown 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 spokesman for the Ondo State governor's office said they would not be giving an official casualty figure for the moment.

But a witness, who gave his name as Abayomi, told AFP 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.

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

- 'Vile and satanic attack' -

Attacks on religious sites are particularly sensitive in Nigeria where tensions sometimes flare between communities in a country with a mostly Christian south and a predominantly Muslim north.

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

Boko Haram jihadist group in the northeast targeted churches in the past as part of Nigeria's conflict that has killed 40,000 and displaced 2 million more.

Kidnapping assaults are common in most parts of Nigeria but mass gun attacks like Sunday's violence are rare in the country's relatively peaceful southwest.

Ondo state governor Rotimi Akeredolu said Sunday's attack was a "vile and satanic attack" and appealed to the security forces to track down the assailants.

The attack comes a day before the ruling APC party starts primaries for its candidate in the 2023 election to replace Buhari, a former army commander who steps down after two terms in office.

Security will be a major challenge for whoever wins the race to govern Africa's most populous country and the continent's largest economy.

Parts of northwest and north-central Nigeria in particular have been increasingly plagued by heavily armed gangs who raid villages and target communities and schools for mass kidnapping attacks.

© Agence France-Presse