DAKAR, Senegal — As rebels swept across Ivory Coast in a rapid advance last week to oust the nation’s strongman, Laurent Gbagbo, hundreds of people were killed in a single town, the United Nations and aid groups said Saturday, in the worst episode of violence during the four-month political crisis that has plunged the country back into civil war.

The exact number of dead was unclear. The United Nations said that 330 people had been killed, while aid organizations put the death toll as high as 1,000. It was also uncertain how many were civilians, and how many were combatants, but Caritas, a Catholic charity whose staff members visited the town, Duékoué, in western Ivory Coast, called it a “massacre.”

The “town was full of bodies,” said Patrick Nicholson, a spokesman for the charity. “They saw bodies in the city, in the bush, mass graves.”

Humanitarian workers did not say who was responsible. But the United Nations said that more than 100 had been killed by Mr. Gbagbo’s fighters, while about 200 had been killed by forces loyal to his rival, Alassane Ouattara, the man recognized by the United Nations, the African Union and other international bodies as the winner of the presidential election last year.