Nigeria Owo church attack: In Ondo, gunmen assassinate Catholic worshippers.

In Ondo State, southwest Nigeria, gunmen have killed churchgoers.

"Only fiends from the nether zone" could have perpetrated this "heinous act," according to President Muhammadu Buhari.

During a Sunday service, armed men stormed into St Francis Catholic Church in Owo. Witnesses stated they opened fire on the gathering before kidnapping a priest and several other churchgoers.

In recent months, Nigeria has seen an increase in violence.

Across the broad country, kidnappings and attacks have been recorded.

Official details for the number of those killed or abducted during the attack on the Christian holiday of Pentecost have yet to be released.

However, a doctor at a local hospital told Reuters that at least 50 bodies had been sent to two hospitals in the town.

Children were among the deceased, according to state politician Ogunmolasuyi Oluwole, who visited the church and hospital.

At least 20 worshipers were killed, according to one witness, Abayomi, who spoke to the AFP news agency.

He stated he was passing through the area when he heard a big explosion and gunshots inside the church, and he saw at least five shooters.

Ondo state governor Rotimi Akeredolu termed it a "vile and diabolical attack" on innocent people in a series of tweets.

He urged people to remain calm and not take the law into their own hands.

In a message released after being briefed at the scene, he continued, "The offenders will be sought down and they will pay for their crimes."

According to a Vatican statement, Pope Francis was praying for the victims who were "painfully stuck in a moment of festivity."

In a statement, President Buhari added, "No matter what, this country will never give in to bad and wicked individuals." He is nearing the end of his two-term president and has been chastised for failing to address the country's security issues.

No one has claimed responsibility for the incident, but armed groups in Nigeria are increasing their aggression, according to the BBC's Chris Ewokor in Abuja. However, the state of Ondo has remained relatively unaffected until recently.

The head of the Methodist Church in Nigeria, along with two other clerics, was kidnapped exactly a week ago in the country's southeast.

According to the Methodist priest, he paid $240,000 (£190,000) to be set free with his accomplices.

Two Catholic priests were kidnapped two weeks ago in Katsina, President Muhammadu Buhari's native state in Nigeria's north. They have not been made public.

Gunmen attacked the important train link between Abuja and Kaduna in northern Nigeria in March, killing at least nine people and kidnapping dozens more, many of whom are still being held captive.