KANO - Fourteen worshippers were killed Wednesday when a suicide bomber attacked a mosque on Nigeria's border with Cameroon, civilian vigilantes told AFP.

The suspected Boko Haram militant blew himself up amid worshippers inside the mosque in Gamboru around 5:00 am (0400 GMT), shortly before morning prayers.

"Fourteen bodies have been pulled out of the rubble," said Umar Kachalla, a civilian militiaman, who said the mosque had been completely destroyed. "Only the muezzin has survived and we believe more bodies are buried under the debris," said Kachalla. "The death toll may likely rise." An hour earlier, a patrol of vigilantes spotted four suspected suicide bombers on the outskirts of the town and arrested one of them after a chase, said a second vigilante, Shehu Mada. "Two of them turned back and fled while the fourth disappeared into the darkness and we believe it was he who attacked the mosque," Mada said.

Boko Haram's eight-year insurgency against the government of Nigeria has spilled into neighbouring Niger, Chad and Cameroon, killing around 20,000 people and displacing more than 2.6 million. In August 2014, the group seized Gamboru, a trading hub along with neighbouring town of Ngala.

Nigerian troops retook both towns in September 2015 with the help of Chadian forces following months-long offensives.

Despite the recapture of the area, Boko Haram fighters continue to launch sporadic attacks, laying ambush on troops and vehicles as well as attacking and abducting farmers.

On Tuesday Boko Haram leader Abubakar Shekau released a video message claiming a series of attacks in the northeast including those in Gamboru.

Meanwhile, hundreds of people stormed Makurdi, the capital of Nigeria's eastern Benue state, on Wednesday protesting the death of 20 farmers allegedly killed by cattle herders, a local civil society group said.

The farmers in Guma and Logo local government areas were reportedly killed by Fulani herdsmen in a series of attacks that took place on Monday and Tuesday. The mainly Muslim nomadic cattle rearers have been clashing with largely Christian farmers over grazing rights in Nigeria for decades.

At a press conference on Tuesday, Benue governor Samuel Ortom said that more than 20 people were killed in the violence, though an official toll has yet to be released.

"People were slaughtered like animals," Ortom was quoted as saying by Nigerian newspaper The Guardian. More than 1,000 people took to the streets of Makurdi and blocked the highway on Wednesday morning, according to Helen Teghtegh, the head of local non-governmental organisation Community Links. "There have been no policies implemented to slow down the attacks made by Fulani herdsmen," she said. "We feel that (President Muhammadu Buhari) being a Fulani man, he's turning a blind eye on the issue."

Teghtegh said another protest was planned for Thursday. The killings usually occur at the end of rainy season between December and March, when the Fulani pastoralists arrive in large numbers to graze their cattle and the farmers start harvesting yams.

But as the country's population explodes - Nigeria is set to become the world's third most populous country by 2050 according to the UN - the battle over land is intensifying.

Hundreds of people were reported dead in Benue state in early 2016 following a week-long clash between herdsmen and farmers.

In November, at least 30 people were killed after farmers attacked herdsmen in the Numan district in eastern Adamawa state.

The violence is a perennial security headache for Nigeria, which has been battling Boko Haram in the northeast since 2009 and a flare-up of militancy in the oil-producing south.

The International Crisis Group, a Brussels-based think-tank, said in a September that some 2,500 people had been killed and tens of thousands were forced from their homes last year.

Such attacks were "becoming as potentially dangerous as the Boko Haram insurgency in the northeast", it added.