Dan Manjang, an adviser to the government of Plateau State, where the attacks took place, said 95 arrests had been made.

Al Jazeera's Yvonne Ndege, reporting from Jos, said the government had confirmed Sunday's death toll at more than 500 and had launched a huge security operation in an attempt to stop the violence from spreading to other states.

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Our correspondent quoted police as saying that the attackers were Muslim Hausa-Fulani herders while the victims were mainly Christians from the Borom community.

But she added that while many people would view the violence in a religious context, people she had spoken to said the violence was about indigenous groups, who are mainly Christian, and migrants and settlers, mainly from the Hausa-speaking Muslim north, competing for access to resources.

The latest violence in the centre of Africa's most populous nation comes at a time of uncertainty for the country, with Goodluck Jonathan, the acting president, trying to assert his authority while Umaru Yar'Adua, the president, remains too sick to govern.

Women and children were among those buried in a mass grave on Monday
The situation is a test of Jonathan's ability to show that he has the power to deploy the police and army as commander-in-chief, and many people will be watching to see how he deals with the situation, our correspondent said.

In a statement after Sunday's attack, Jonathan's office said he had "directed that the security services undertake strategic initiatives to confront and defeat these roving bands of killers".

Mohammed Lerama, a police spokesman, said the official death toll stood at 55 so far, but Gregory Yenlong, Plateau State's Commissioner for Information, said: We are estimating 500 people killed but I think it should be a little bit above that."

Yenlong added that "soldiers are patrolling and everywhere remains calm", but security officials have been criticised for failing to prevent another outburst of violence just weeks after hundreds died in Muslim-Christian clashes.

The violence in the three, mostly Christian, villages on Sunday appeared to be reprisal attacks following the January unrest in Jos when most of the victims were Muslims, Robin Waubo, a Red Cross spokesman, said.