Forces loyal to Ivory Coast’s president-elect pushed their two-week-long military offensive deeper into the country’s main city Saturday, threatening the entrenched incumbent’s hold on power, as U.N. and aid officials claimed that several hundred people had been massacred in a western town last week.

Estimates of the dead in the contested town of Duekoue were hard to pin down, with figures ranging from more than 300 to 1,000 people killed. The International Committee of the Red Cross said that more than 800 civilians had been killed in “intercommunal” violence in the town, where tens of thousands of Ivorian civilians have been displaced by fighting between the forces of Alassane Ouattara, the internationally recognized winner of a November election, and Laurent Gbagbo, who refuses to step down.

“We are shocked by the brutality and scale of this act,” said Dominique Liengme, the Red Cross’s representative in Ivory Coast.

The United Nations put the number lower, saying that 330 people had been killed in the town by rival combatants. Hamadoun Toure, spokesman for the U.N. mission in Abidjan, said there was evidence that Ouattara’s forces were responsible for the deaths of at least 230 civilians and Gbagbo’s fighters for at least 100. Patrick Nicholson, a spokesman for the Catholic aid agency Caritas, said that an agency team in the town last week on a routine aid mission had found “a lot of dead bodies.”

“We estimate between 800 and 1,000 dead,” Nicholson said in a telephone interview from Rome. “They were primarily killed by gunshot, though some of the wounds were made by machetes. I don’t think they were killed in crossfire.”

As the United Nations and humanitarian organizations sought to tally the dead in Duekoue, rival forces battled for control of the commercial capital, Abidjan, where key installations seized Friday by Ouattara’s forces, including the national radio station, fell back into the hands of Gbagbo’s troops Saturday morning.

Alain Le Roy, the U.N. undersecretary general for peacekeeping, said in an interview Saturday that Gbagbo’s forces were firing indiscriminately in urban neighborhoods.

“The situation has never been this tense,” said Toure. “We have heavy weapons being used against the population. It’s like a ghost town. You won’t find a soul outside.”

The violence in Ivory Coast has its most recent roots in the country’s disputed Nov. 28 runoff presidential election. Gbagbo lost to Ouattara in a U.N.-certified vote that was largely backed by foreign governments, but he has refused to cede power. Several months of high-level diplomatic efforts, combined with financial sanctions, have failed to budge him.

U.N. officials and human rights groups say that in an effort to consolidate power, Gbagbo’s forces have perpetrated serious human rights abuses against civilians suspected of supporting Ouattara, as well as attacking U.N. installations and personnel.

Last month, a coalition of former rebel groups, police and soldiers mounted a military offensive against Gbagbo, beginning in the rebel stronghold in northern Ivory Coast. The armed movement met with relatively little resistance as it seized town after town, capturing control of the administrative capital, Yamoussoukro, last week, and bringing the fight to Gbagbo’s stronghold in Abijdan on Friday.

But Ouattara’s forces have also been accused of carrying out reprisal killings and extrajudicial executions of prisoners during their march to the capital.

“We have credible reports of serious abuses being committed by Ouattara’s side,” said Corinne Dufka, a Human Rights Watch researcher based in Dakar, Senegal. “It’s raising very serious concerns.”

Human Rights Watch and the U.N. human rights agency have appealed to Ouattara to take firmer steps to ensure that his troops conduct themselves lawfully and avoid reprisal killings. On March 31, Ouattara had urged Ivorians to join the armed movement against Gbagbo, saying diplomacy had run its course. But he also urged the commanders “to prevent the looting of property and atrocities on people.”

Le Roy said that there clearly had been “killings by the hundreds” in Duekoue, long the scene of inter-ethnic violence between tribes supporting the rival camps.

“I understand it has mostly been done by forces close to Ouattara,” Le Roy said of the massacre, adding that Ouattara’s camp has acknowledged killing a large number of people there but claiming they were members of an armed militia, not civilians.

He also said Ouattara has agreed to an independent investigation of the killings.