Many of the victims were cut with machetes, doctors said
At least 100 people have been reported killed in suspected religious clashes near the central Nigerian city of Jos.
Witnesses said several villages just outside of the city were attacked simultaneously overnight.
Acting President Goodluck Jonathan has put security forces in central Nigeria on full alert.
In January hundreds of people were killed in riots in Jos, which lies between the mainly Muslim north and the more Christian south.
Ethnic and religious riots also broke out in 2008, killing hundreds.
The attack happened before dawn on Sunday morning when gangs of men descended on several communities, centred on the village of Dogo-Nahawa, and attacked people with machetes, reports say.
A resident of Dogo-Nahawa said the attackers had fired guns as they entered the village.
"The shooting was just meant to bring people from their houses and then when people came out they started cutting them with machetes," Peter Jang told Reuters.
JOS, PLATEAU STATE
Deadly riots in 2001, 2008 and 2010
City divided into Christian and Muslim areas
Divisions accentuated by system of classifying people as indigenes and settlers
Hausa-speaking Muslims living in Jos for decades are still classified as settlers
Settlers find it difficult to stand for election
Divisions also exist along party lines: Christians mostly back the ruling PDP; Muslims generally supporting the opposition ANPP
An aid worker with the Christian charity Stefanus Foundation, Mark Lipdo, said at least 100 people had been killed.
He told the BBC he went to the villages of Zot and Dogo-Nahawa after daylight on Sunday and recorded the names of 77 victims and said there were at least two dozen more bodies.
"We saw mainly those who are helpless, like small children and then the older men, who cannot run, these were the ones that were slaughtered."
He said Zot had been almost wiped out.
Other witnesses said they had also seen at least 100 bodies and a Plateau state official told Reuters news agency that more than 300 people had died.
A doctor at a hospital in Jos told news agencies that victims had been cut by machetes and burnt.
The military, which already has a presence in Jos, has sent troops to Dogo-Nahawa.
"The acting president has placed all the security forces in Plateau and neighbouring states on red alert so as to stem any cross-border dimensions to this latest conflict," Mr Jonathan's office said in a statement quoted by Reuters news agency.
He also ordered those behind the violence to be found.
Analysts say the attack seems to be in reprisal for the clashes between Christians and Muslims in January, which claimed the lives of at least 200 people and displaced thousands of others.
Hundreds of people have fled from Jos in the aftermath of the fighting, the Red Cross says.
Robin Waudo, a spokesman for the International Committee of the Red Cross, told the BBC his volunteers were assisting people wounded in the latest fighting.
"We know that late this morning there was some fighting in the south part of the city and it seems like there are reprisal attacks from what happened a few weeks ago," he said.