for years but has grown increasingly aggressive in recent months, including a campaign of forced conversions targeting both Christians and Muslims, officials said.
"A detachment of 60 police... came under attack from members of Ombatse cult in an ambush," Nasarawa state police chief Abayomi Akeremale said of the attack on Tuesday in the village of Elakyo, some 10 kilometres (six miles) outside the state capital Lafia.
"The Ombatse gunmen opened fire on our men, killing 23 and burning them," he told AFP, adding that 17 officers remain missing.
Nasarawa roughly falls on the dividing line between Nigeria's mostly Christian south and predominately Muslim north.
One of the state's major ethnic groups, the Eggon, is divided between the two faiths, but also has a history of links to pagan movements.
Ombatse, which means "time has come" in Eggon, has a significant presence, although its strength has typically been hard to estimate.
The group has identified alcohol consumption and adultery as some of the sins it seeks to eliminate.
Nasarawa's Commissioner for Information, Hamza Elayo, told AFP that the authorities had no issue with Ombatse until the movement's aggressive turn.
"Everybody has right to freedom of religion, but when people go about forcing their creed on others in a violent way it becomes unacceptable," he said.
Elayo and the police chief said the number of reported forced conversions has surged in recent weeks, prompting the security forces to move against a self-proclaimed priest and others.
"We decided to send our men to the area to arrest members of Ombatse including their priest," Akeremale said.
"(They) have been going to churches and mosques initiating people into their cult by forcefully administering an allegiance oath to unwilling people."
Elayo said there were indications that the movement's change in conduct had political motivations.
The Eggon, according to Elayo, have been pushing for a member of their ethnic group to succeed the current state governor, Tanko Al-Makura, and certain Eggon leaders have reached out to the Ombatse to fight for their cause.
"It is obvious they are being sponsored by some ambitious politicians...The security agencies have been closing in on such politicians but I don't want to mention names," he said.
Access to massive resources are at stake in Nigerian state elections and contests across the country have turned violent.
Political and ethnic groups have also mobilised private militias to advance their agendas.
The male members of Ombatse are said to dress only in black, while women are reportedly barred from many of the major ceremonies.
Until recently, incidents of violence were limited.
Last November, Ombatse gunmen were reported to have shot dead three security personnel in a shootout with troops who stormed a shrine during an initiation ceremony.
Days later, in an apparent reprisal for the raid on their shrine, purported Ombatse gunmen killed 10 members of a rival group while destroying several homes.
The state government in December launched a probe into the activities of the Ombatse following the unrest.
Nigeria, Africa's most populous country, includes some 250 ethnic groups.
While Christianity and Islam are dominant, various other religious and spiritual movements are present across the country.
Nigerian media frequently carry reports of crimes related to the occult, but the details are often murky.