As many as 500 people may have been killed in attacks on Sunday on two predominantly Christian villages near the Nigerian city of Jos.
The worst hit was the village of Dogo Nahawa, where Muslims reportedly set fire to homes before killing villagers as they attempted to flee.
Officials raised the number of dead from their previous estimate of around 100. They said the attackers were armed with machetes.
One reporter with the independent Nigerian news network Channels counted more than 200 bodies, mainly women and children, according to The Guardian. Some of the bodies had machete wounds while others were charred.
Locals believe the attacks were carried out in revenge for the killing of hundreds of Muslims in January. The area has been blighted for years by tensions between the Muslim Fulani ethnic group and the Christian Berom ethnic group.
Peter Gyang, who lost his wife and two children in the attacks, was quoted by Agence France Presse as saying: "Gunshots were fired just to scare people out of their houses only to be macheted as they fled into the bush.
"The operation started around 3:00 a.m. and lasted till 6:00 a.m. and there were gunshots, but we did not see a single policeman," Gyang continued. "We no longer have confidence in the security agencies."
Acting President Goodluck Jonathan has placed security forces in Plateau State and neighboring states on high alert in a bid to stop the violence from spreading.
Nigeria, the most populous country in Africa, is about evenly split between Muslims in the north and Christians in the south. More than 13,500 people have died in religious or ethnic clashes since the end of military rule in 1999, according to Human Rights Watch.