Mark Lipdo of the Stefanos Foundation said he witnessed the massacre
Some 500 people, including many women and children, are now reported to have died in a weekend religious clash near Nigeria's city of Jos, officials say.
The figure had previously been put at about 100 but it is always difficult to verify casualties. Local officials said dozens of arrests had been made.
They said three mainly Christian villages near Jos were attacked from nearby hills by people with machetes.
There is a long history of local tension between Muslims and Christians.
The attacks are said to have been in revenge for the killing of several hundred people around Jos in January.
Although sectarianism is blamed for such clashes, correspondents say poverty and access to resources such as land often lie at the root of the violence.
Acting President Goodluck Jonathan has put security forces on alert to stop the flow of weapons to the area.
The AFP news agency reports that troops and military vehicles have entered the villages, which are now said to be calm.
An adviser to the Christian-dominated Plateau state government, Dan Manjang, told AFP: "We have been able to make 95 arrests but at the same time over 500 people have been killed in this heinous act."
Another Plateau state official, Gregory Yenlong, urged people to "remain calm and be patient as the government steps up security to protect lives and property in this state".
Many of the dead in the villages of Zot and Dogo-Nahawa are reported to be women and children.
Mark Lipdo, from the Christian charity Stefanos Foundation, said the village of Zot had been almost wiped out.
He said: "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."
A resident of Dogo-Nahawa said the attackers had fired guns as they entered the village before dawn on Sunday in defiance of a curfew.
"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 news agency.
Some witnesses said villagers were caught in fishing nets and animal traps as they tried to escape and were then hacked to death. Mud huts were also set on fire.
Mass burials took place on Sunday and scores more bodies were laid out in the streets of the three attacked villages, awaiting further burials on Monday.
Figures given for the death tolls in the religious clashes have varied widely, sometimes to achieve political ends or to reduce the risk of reprisals, or simply because victims are buried quickly.
Jos lies between the mainly Muslim north of Nigeria and its largely Christian south.
Analysts say the latest 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 International Committee of the Red Cross says.
The religious clashes represent a challenge for the acting leader, Mr Jonathan. He formally took over last month from President Umaru Yar'Adua, who has a heart problem.
Mr Yar'Adua returned from three months of treatment in Saudi Arabia two weeks ago but has still not been seen in public.
Mr Jonathan is a Christian from the Niger Delta and President Yar'Adua is a Muslim northerner.
Last week their ruling People's Democratic Party (PDP) said it would field a northerner as its candidate in next year's presidential election, ruling out Mr Jonathan.
Under an unwritten power-sharing agreement within the PDP, the party's candidates for the presidency alternate between north and south after every two terms in office.
Mr Yar'Adua became president in 2007, succeeding Olusegun Obasanjo, a southern Christian who served two terms.
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