DAKAR, Senegal  Government security forces were implicated in the deaths of at least 90 of the hundreds of people killed in religious violence in the central Nigerian city of Jos last month, Human Rights Watch said Saturday.

At least 400 people died when fighting broke out between Muslim and Christian gangs in the city on Nov. 28 and 29 after a dispute about local elections. Each side accused the other of rigging the vote.

Muslims and Christians mingle uneasily in the so-called middle belt region of Nigeria, and tension frequently flares along religious, ethnic and political fault lines.

Initial accounts of the killing in Jos indicated that the gangs had set upon each other’s neighborhoods, burning churches, homes, businesses and mosques.