Nigerian authorities say they have arrested scores of people in connection with attacks near the central city of Jos that left more than 200 people dead.
"We have been able to make 95 arrests but at the same time over 500 people have been killed in this heinous act," Dan Manjang, an adviser to the Plateau State government told the AFP news agency.
State radio also reported that 500 people had been slaughtered in Sunday's raid on three villages on the fringes of Jos, although there has been no independent verification of the death toll.
Yvonne Ndege, Al Jazeera's correspondent in Abuja, Nigeria's capital, said at least 19 Muslim Hausa-Fulani men had been arrested 30km from Jos, where Sunday's three-hour systematic orgy of violence took place.
"They were found to be carrying hunting guns, axes, spears and knives and the feeling is that they were linked to this attack," she said.
The reported arrests come as security officials faced criticism for failing to prevent another outburst of sectarian violence only weeks after hundreds died in Muslim-Christian clashes.
Troops have been deployed in the volatile region to rein in gangs that rampaged across villages near the flashpoint central city, slaughtering women and children, including a four-day-old infant.
The violence in three mostly Christian villages 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.
Our correspondent quoted police as saying that the attackers were Muslim Hausa-Fulani herders while the victims were mainly predominantly Christians from the Borom community.
"This really is a test for his [president] ability to show that he does have the powers to deploy the police and army as commander-in-chief, and many people will be watching exactly to see how he deals with this," Ndege said.
The office of Goodluck Jonathan, Nigeria's acting president, said he had "directed that the security services undertake strategic initiatives to confront and defeat these roving bands of killers", blamed for "causing considerable death and injury".
Officials from the central government were holding an emergency meeting, said our correspondent, adding that there was now a heavy police presence in the area to prevent any reprisal attacks.
According to a Red Cross official, at least two other nearby communities were also targeted, in an area close to where sectarian clashes killed hundreds of people in January.
It was not immediately clear what triggered the latest unrest, but four days of sectarian clashes in January between mobs armed with guns, knives and machetes left hundreds of people dead in Jos, which lies at the crossroads of Nigeria's Muslim north and predominantly Christian south.
The tension appears rooted in resentment between indigenous, mostly Christian groups, and migrants and settlers from the Hausa-speaking Muslim north, all vying for control of fertile farmlands.
"Part of the problem is that there's a feeling that Jos is being dominated by migrant communities from nothern Nigeria...," our correspondent said.
Felix Onuah, a freelance journalist, told Al Jazeera that he had spoken to the information commissioner of the Plateau State who said that the attack "was nothing but ethnic cleansing".
"It's a revenge," he said, adding that the ethnic group that retaliated was adversely affected during the January violence.