DAKAR, Senegal — Officials and human rights groups in Nigeria sharply increased the count of the dead after a weekend of vicious ethnic violence, saying Monday that as many as 500 people — many of them women and children — may have been killed near the city of Jos, long a center of tensions between Christians and Muslims.

The dead were Christians and members of an ethnic group that had been feuding with the Hausa-Fulani, Muslim herders whom witnesses and police officials identified as the attackers. Officials said the attack was in reprisal for violence in January, when dozens of Muslims were slaughtered in and around Jos, including more than 150 in one village.

Early Sunday, the attackers set upon the villagers with machetes, killing women and children in their homes and ensnaring the men who tried to flee in fishnets and animal traps, then massacring them, according to a Nigerian rights group whose investigators went to the area. Some homes were set on fire.

The latest attacks were “a sort of vengeance from the Hausa-Fulani,” said the Rev. Emmanuel Joel, of the Christian Association of Nigeria in Jos. After the January attacks, “the military watched over the city, and neglected the villages,” he said.