West African leaders said they would use "legitimate force" to remove Laurent Gbagbo from power in Ivory Coast if he does not agree to step down peacefully, a call that comes amid rising fears of violence.

At least 173 deaths have been confirmed in violence over the presidential vote, and the U.N. is warning the number could be greater since it has been unable to investigate all the allegations.

Masked gunmen with rocket launchers have blocked access to what officials believe may be a mass grave site in Ivory Coast, the United Nations said.

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

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 presidential runoff vote was held nearly a month ago.

Even the top U.N. envoy in the country was stopped at gunpoint while trying to look into reports of human rights abuses, the U.N. deputy human rights commissioner in Geneva said Thursday.

United Nations peacekeeping chief Alain Le Roy called on Gbagbo to keep his pledge not to resort to violence.

"We hope for a peaceful solution to the crisis," Le Roy said in an interview with UN Radio.

Alain Toussaint, an adviser for Gbagbo, has said that he didn't believe 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 from the U.N., U.S., former colonizer France, the European Union and the African Union. The international community recognizes Alassane Ouattara as the winner, though Gbagbo maintains control of the national military.

James Gbeho, president of the regional bloc ECOWAS — the Economic Community of West African States — said the group of West African leaders was making an "ultimate gesture" to Gbagbo to urge him to make a peaceful exit.

The 15-nation regional bloc of West African states made the decision following a six-hour emergency summit in Abuja, Nigeria, on Ivory Coast as worries mounted that the country that suffered a 2002-2003 civil war could return to conflict.

Gbeho said the bloc would send in a high-level delegation to meet with Gbagbo, and tell him to step down.

Gbagbo claimed victory in the presidential election only after his allies threw out half a million ballots from Ouattara strongholds in the north, a move that infuriated residents there who have long felt that they are treated as foreigners in their own country by southerners.

"In the event that Mr. Gbagbo fails to heed this immutable demand of ECOWAS, the Community would be left with no alternative but to take other measures, including the use of legitimate force, to achieve the goals of the Ivorian people," a statement from ECOWAS said.

Gbeho said chief of defense staffs should meet "to plan future actions, including provision of security along the Ivory Coast-Liberia border, in the event that their message is not heeded."

Amid the rising concerns over violence, the United States has said it and other countries are discussing ways to help quell the postelection violence.

"We are in discussions with other regional countries to see if there are ways in which we can reinforce the U.N. peacekeeping force," spokesman P.J. Crowley told reporters Wednesday. "It could be that that kind of reinforcement could be another way to send a clear message to President Gbagbo."

Crowley declined to name the countries that have been contacted but noted that Nigeria is a major troop contributor to West African peacekeeping forces. He also noted that France has interests in Ivory Coast, a former French colony where at least 13,000 French citizens reside.

A Nigerian military spokesman said that military intervention into another country could only be decided by the president.

There has been little international interest so far in a military intervention in Ivory Coast. The United States and the European Union are imposing sanctions targeting Gbagbo, his wife and political allies. Hundreds of U.N. peacekeepers have been protecting the hotel where Ouattara is based.

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

"Serious human rights violations have been recorded all over," he 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 has asked the International Criminal Court to send a team to investigate in the coming days.

Ouattara also called on the army and the U.N. to protect civilians.

The U.S. State Department has ordered most of its personnel to leave because of the deteriorating security situation and growing anti-Western sentiment, and former colonizer France is also urging its citizens to leave. A youth leader accused of inciting a pro-Gbagbo group that has led violent attacks against foreigners in the past has called for a demonstration Wednesday.

Meanwhile, Ouattara is trying to assert control over state institutions. State television, which had been controlled by Gbagbo, was yanked from the air in most cities outside Abidjan late Thursday, and Gbagbo's access to state funds also has been blocked.

Ouattara's allies hope the move by the West African economic and monetary union late Thursday to give Ouattara's government signing privileges on state accounts will set the stage for mass defections if Gbagbo cannot pay civil servants and soldiers in the military.

While Ouattara has the backing of the international community, Gbagbo still controls the country's military — and had dominated state media until late Thursday.

Ivory Coast was once an economic hub because of its role as the world's top cocoa producer. The 2002-2003 civil war split the country into a rebel-controlled north and a loyalist south. While the country officially reunited in a 2007 peace deal, Ouattara draws his support from the northern half of the country, where he was born, while Gbagbo's power base is in the south.

___

Associated Press writer Bashir Adigun in Abuja, Nigeria contributed to this report.