I. Coast violations may be crimes against humanity-UN

* Killings, detentions alleged to continue, torture feared

* U.N. experts denounce horrendous acts, demand justice

GENEVA, Dec 31 (Reuters) - Killings, disappearances and arbitrary detentions are continuing in Ivory Coast and some violations may amount to crimes against humanity, United Nations human rights experts said on Friday.

In a joint statement, the independent investigators voiced alarm that hundreds of Ivorians have been rounded up. Some are being held incommunicado and are believed to face a greater risk of tortur.

World leaders have stepped up pressure on Laurent Gbagbo to step down and cede power to Alassane Ouattara, who is widely recognised to have won a presidential election last month. Gbagbo&${esc.hash}39;s camp has rejected the U.N.-certified results that declared Ouattara the winner.

"When committed in certain circumstances, enforced disappearances amount to a crime against humanity," said the U.N. working group on enforced or involuntary disappearances. "Those who have perpetrated such horrendous acts shall be held accountable."

Victims of enforced disappearances, including the relatives of the disappeared, have the right to justice, redress, truth and adequate reparation, said the group, composed of five independent experts.

Separately, U.N. experts on arbitrary detention said some who had been arbitarily arrested and taken to "illegal places of detention" were being held without charge. These amounted to "heinous violations of international human rights law".

The experts report to the U.N. Human Rights Council, which held a special session on Ivory Coast on Dec. 23 and condemned the violence there.

Juan Mendez, U.N. special rapporteur on torture, called for investigations into allegations of torture or cruel and inhuman treatment and said authorities must prevent further abuses.

Perpetrators of torture should be brought to justice and severely punished, he said.

In New York on Thursday, U.N. advisers expressed grave fears about ethnic violence in Ivory Coast and Abidjan&${esc.hash}39;s new ambassador to the world body warned that the West Afrifcan nation was on the "brink of genocide". [ID:nLDE6BT19V] (Reporting by Stephanie Nebehay; editing by Noah Barkin)