ABIDJAN, Ivory Coast - Reports of dozens of bodies being dumped near a large forest first emerged as human-rights groups warned that security forces loyal to incumbent leader Laurent Gbagbo were abducting political opponents after the disputed presidential election.

Now the United Nations thinks that up to 80 bodies may have been moved to a building nestled among shacks in a pro-Gbagbo neighborhood. Investigators have tried to go there several times, and even made it as far as the building's front door before truckloads of men with guns showed up and forced them to leave.

Simon Munzu, head of the U.N. human-rights division, urged security forces Thursday to allow investigators inside. Gbagbo's government has repeatedly denied the existence of mass graves after violence over the disputed election runoff that has left at least 173 people confirmed dead already.

"We would be the very first to say that these stories are false if they turn out to be false," Munzu said. "Our findings on the matter and their announcement to the world would have a greater chance of being believed than these repeated denials."

Human-rights groups accuse Gbagbo's security forces of abducting and torturing political opponents since the Nov. 28 vote, which the United Nations said Gbagbo lost. U.N. investigators have cited dozens of reported cases of disappearances, and nearly 500 arrests and detentions.

The United Nations has said that security forces accompanied by masked men with rocket launchers also prevented U.N. personnel from reaching the building. Munzu said witnesses had said 60 to 80 bodies were believed to be inside.

A second mass-burial site is believed to be near Gagnoa in the country's interior, the United Nations said.

The reports raise new concerns about rights abuses as Ivory Coast's neighbors discuss how to remove Gbagbo from power.

Meanwhile, a fiery member of Gbagbo's cabinet urged supporters to seize a hotel where Alassane Ouattara, the internationally recognized winner of the election, has been organizing a shadow government under U.N. protection.

The hotel is protected by 800 U.N. peacekeepers and hundreds of rebels loyal to Ouattara.