ABUJA, Nigeria — Clashes between armed herdsmen and farmers in Nigeria are escalating to increasingly violent episodes as a battle for scarce resources stirs long-held tensions over religion and ethnicity.

By some estimates the clashes have taken more than 500 lives this year.

In recent days, at least 86 people were killed in several villages in Plateau State in the middle of the country, among the deadliest of the episodes.

On Friday and Saturday night dozens of suspected armed herders, who are Muslim and of the Fulani ethnicity, descended from surrounding hills into several villages, opening fire, burning homes and shooting to death some people, most of them Christian and of the Berom ethnicity, as they slept.

The killings immediately triggered reprisals as young people from the villages set up road blocks and killed anyone suspected of being Muslim and Fulani. One police commissioner in the area said at least five people died at checkpoints.