“They killed as many men as they could,” the 65-year old grandmother said, including three of her relatives. “A young boy pointed his weapon at me. I still can’t comprehend why he didn’t pull the trigger.”

The massacre at Aya Mbalom village—the latest clash in what has become the deadliest conflict to roil Africa’s most-populous nation—comes after a year of attacks and reprisals that have left more than 1,500 people dead and pushed more than half a million from their homes across Nigeria’s most-fertile farming regions.

The clashes are the result of a battle over dwindling supplies of farmland between mainly Christian farming communities and mainly Muslim herdsmen who have for centuries lived in relative harmony.

Fighting has intensified in recent months after the government passed new laws to halt grazing in a bid to stop the deadly clashes and raise agricultural output.

Officials fear the conflict could intensify ahead of elections next year that are considered a referendum on how President Muhammadu Buhari has addressed violence in the country, including the war against the government being waged by Islamist insurgency Boko Haram. President Donald Trump raised the issue of Christian killings in this month’s White House meeting with Mr. Buhari.

Nigeria’s media and Christian politicians say the murders are the work of “killer herdsmen”: nomadic cattle farmers from the Fulani ethnic group, armed with machine guns and Kalashnikovs. Fulani leaders say they are defending themselves from farming communities that have formed militias to hunt and kill them.

Nigeria’s government has said criminal elements have penetrated both sides. Senior security officials say some Fulani groups have hired defectors from Boko Haram to carry out killings, a claim the herdsmen deny.

Nigeria’s government this month declared a national-security emergency and pledged to deploy specialized agricultural-protection divisions across thousands of miles. Farmers have been angered by what they see as the tepid response from Mr. Buhari—himself of Fulani stock—who has called the killings “regrettable” and blamed bandits trained by Libya’s former dictator, Moammar Gadhafi.

Some analysts say the clashes could foreshadow broader resource wars across West Africa’s Sahel—the semiarid region bedeviled by a confluence of rising jihadist activity and surging migration—and around the continent.

“Confrontation between the Fulani and other Nigerian groups could have regional repercussions, drawing in fighters from neighboring countries,” the International Crisis Group said in a report: “These clashes are becoming as dangerous as the Boko Haram insurgency.”

The herdsman communities—concentrated in the Sahel but moving their herds across thousands of miles to Central African Republic from Senegal—have traditionally taken cattle south during dry season to graze—and in return fertilize farmers’ land.

But longer dry seasons, the expansion of the desert and one of the world’s fastest-growing populations, has destroyed that equilibrium. In northern states such Sokoto, Katsina, Bauchi and Kano, as much as 75% of land is becoming desert, according to the International Crisis Group, forcing herders further south, often heavily armed. There they have met settled farmers who are harvesting more land as the pressure to feed a population estimated to swell to 400 million by 2050, from an estimated 186 million, larger than the U.S.

Diplomats in the capital Abuja said the land conflict could be politically toxic for the government.

“This is extremely serious—it could make or break the elections. Buhari himself is at risk,” a senior Western official said. “He has been so late to address these issues.”

For now, the crisis is reverberating across Nigeria’s fertile states, known as the middle belt.

In Benue’s capital Makurdi, checkpoints have been erected on roads to affected villages. The production of food—including cassava, maize and soy—has collapsed, local officials said. Villagers are organizing into local defense forces.

Local officials say Fulani herdsmen are occupying farms in more than 70 villages, sending thousands fleeing and rendering more than half of the state inaccessible. On Makurdi’s outskirts, two camps now house 30,000 displaced people, with more arriving daily. At one camp, housed in a dilapidated primary school, two toilets serve 10,000 people.

Benue Gov. Samuel Ortom warns that the sectarian dynamic of attacks on churches risked moving the conflict into dangerous new territory.

“Islamic State, Boko Haram or Fulani mercenaries, they are all working toward achieving one agenda—which is invading and taking over our land,” he said.

Fulani groups said the killings are the work of marginal criminal elements and warn that Mr. Ortom’s rhetoric is evidence of prejudice against them.

“The conflict has become uncontrolled, religious and sectarian. Politicians are using it for their own purposes and the language recalls Rwanda before the genocide,” said Usman Ngelzerma, head of Miyetti Allah Cattle Breeder’s Association, Nigeria’s largest Fulani advocacy group, as he looked through a book showing bludgeoned corpses of slain Fulanis. “Above all, the media is biased against us. They are uniting each and every community in hatred of our people.”

Mr. Ngelzerma said the 20 million cattle farmers, who own 50 million livestock, demand a repeal of the new laws restricting grazing.

Civil-society groups have proposed establishing grazing reserves that could provide pasture for some 20 million cattle.

Nigerian Nobel laureate Wole Soyinka has called for international intervention, warning the killings could spiral into the kind of ethnic cleansing seen in Yugoslavia in the 1990s.

On Tuesday, Benue’s farming community buried the two dead pastors in an emotionally charged ceremony attended by 20 bishops and thousands including Vice President Yemi Osinbajo. The site chosen for the burial is heavy with symbolism—a 40-foot cross atop a hill overlooking the farmland.

Mr. Osinbajo urged the community to forgive, promising that the government would apprehend the culprits and rebuild damaged communities.

Iornongu Geoffrey, the 63-year-old pastor administering the slain priests’ dioceses, echoed the community’s anger.

“These priests were killed in their complete dressing. These people knew what they were doing,” he said. “The government has to give us arms so we can have courage.”