ABIDJAN, Ivory Coast - Masked gunmen with rocket launchers are blocking access to what officials believe may be a mass grave in Ivory Coast, the United Nations said, as concerns grow that armed conflict could return to the West African nation, which suffered a 2002-03 civil war.

The U.N. reported that heavily armed forces allied with Laurent Gbagbo, and joined by masked men, were preventing people from getting to the village of N'Dotre, where "allegations point to the existence of a mass grave," the global body said.

The U.N. did not elaborate on the possible victims, though it has expressed concerns about hundreds of arrests and dozens of cases of torture and disappearance during the political turmoil since the Nov. 28 presidential runoff vote.

"As the violence goes on, the number of dead, wounded, and missing persons is increasing rapidly," U.N. deputy spokesman Farhan Haq said late Thursday.

Alain Toussaint, an adviser to Gbagbo, has said he did not believe that soldiers or people close to Gbagbo would carry out the acts of violence that have been reported.

Gbagbo has refused to step down from the presidency despite international calls for his ouster, and maintains control of the national military.

The international community recognizes opposition leader Alassane Ouattara as the election winner.

Ouattara condemned the violence in a speech Friday at the Golf Hotel, where he has been holed up since the election, after journalists were helicoptered in by the U.N. to cover his remarks.

"Serious human-rights violations have been recorded all over," Ouattara said. "During the curfew, people were kidnapped and killed by Republican Guards and military police accompanied by mercenaries and foreign militiamen."

"I will do everything so that these atrocities don't happen again," he said, specifying that he had asked the International Criminal Court to send a team to investigate. Ouattara also called on the army and the U.N. to protect civilians.

Also Friday, West African leaders threatened to use "legitimate force" to remove Gbagbo if he does not give up power, said James Gbeho, president of the Economic Community of West African States, after an emergency summit on Ivory Coast held in Abuja, Nigeria.