Officers report that people armed with guns targeted a congregation in Benue State - an area plagued in recent years by clashes between mobile herdsmen and settled farmers, often over land use.
The latest incident struck St Ignatius' Catholic Church in the village of Ayar Mbalom, Gwer East. According to a police official quoted by Reuters news agency, it happened at around 6am local time on Tuesday.
Responding on social media to the attack, Nigeria's President Muhammadu Buhari tweeted: "I extend my sincere condolences to the Government and people of Benue State, the Mbalom community, and especially the Bishop, priests & members of the St Ignatius' Catholic Church, whose premises was the unfortunate venue of the heinous killings of worshippers & two priests by gunmen."
Hundreds of people have died in violence in Benue and across the so-called Middle Belt of Nigeria so far this year - with disputes over grazing rights and access to fertile land fuelling tensions.
Both herdsmen - who are mainly Muslim and from the Fulani ethnic group - and the settled farmers - who are predominantly Christian - have carried out attacks.
Pledging to ensure the perpetrators of Tuesday's attack are "brought to justice", President Buhari added: "This latest assault on innocent persons is particularly despicable.
"Violating a place of worship, killing priests and worshippers is not only vile, evil and satanic, it is clearly calculated to stoke up religious conflict and plunge our communities into endless bloodletting."
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