(Adds details throughout on abuse)

By Claudia Parsons

UNITED NATIONS, July 20 (Reuters) - The United Nations is investigating allegations of widespread sexual abuse by hundreds of Moroccan peacekeepers serving in Ivory Coast and has summoned Rabat's diplomats to respond, U.N. officials said on Friday.

U.N. officials, speaking on condition of anonymity, said the investigation involved Moroccan soldiers having sex with a large number of underage girls. The world body took the highly unusual step of confining the entire battalion of 800 troops to barracks.

"An internal investigation by the United Nations Mission in Cote D'Ivoire has revealed serious allegations of widespread sexual exploitation and abuse by a U.N. military contingent serving in Bouake," a U.N. statement said.

Over the last few years as peacekeeping has expanded, reports of abuse have mounted in various African nations, especially the Democratic Republic of the Congo, despite the "zero-tolerance" policy declared by the United Nations.

"If the allegations are proven to be true, it's entirely unacceptable that U.N. peacekeepers would behave in this manner," one U.N. peacekeeping department official said.

The Ivory Coast mission numbers just over 9,000 uniformed personnel from more than 40 countries. Moroccans make up the bulk of the force in Bouake, a rebel stronghold in the northern Ivory Coast, with some Bangladesh police, Pakistani engineers and Ghanaian medical personnel.

The Moroccan U.N. mission had no immediate response to the allegations.

The U.N. statement said a full investigation was underway.

"But due to the serious nature of the allegations, the United Nations has taken the decision to suspend all activities of the contingent and has cantoned the unit within its base," the statement said.

The U.N. Security Council voted this week to extend the mandate of peacekeeping forces in Ivory Coast until January to help create peaceful conditions for elections that have been repeatedly delayed.

The peacekeepers, backed by troops from former colonial power France, are in Ivory Coast to support a peace process that was revived in March by an agreement between President Laurent Gbagbo and rebel leader Guillaume Soro.

The United Nations ignored sexual exploitation by peacekeepers and other field staff for decades, launching a crackdown only in recent years after reports of abuse surfaced in the Congo.

A 2005 U.N. report said soldiers should be punished for any sexual abuse, their pay docked and a fund set up to assist any women and girls they impregnated. But member nations have not agreed.

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