BANGUI (Reuters) - A top U.N. official warned on Wednesday of "ethnic-religious cleansing" in the Central African Republic, as peacekeepers uncovered a mass grave at a military camp occupied by Seleka rebels in the capital Bangui.

The atrocity is the latest in a that has gripped the former French colony since March, when the mainly Muslim rebels seized power. Their campaign of rape, torture and executions against the majority Christian population triggered inter-religious violence which has displaced a million people.

A Reuters witness saw at least a dozen decomposed bodies at the military camp in the 200 Villas neighbourhood of Bangui, where Seleka fighters have been stationed for several months.

The bodies had been stuffed into a large underground chamber, possibly a septic tank.

Pastor Antoine Mboa Bogo, head of the local Red Cross, confirmed the existence of the grave and said his staff would return to the camp on Thursday to determine the number of dead. It is not known who committed the killings.

Violence has worsened since Seleka leader Michel Djotodia resigned as president last month under international pressure.

The subsequent withdrawal of Seleka troops from the country's south set the stage for reprisals by mostly Christian "anti-balaka" militia against Muslims.

France has deployed around 1,600 troops to support an African Union peacekeeping force of 6,000 soldiers but they have been powerless to halt the communal violence.

On a visit to Bangui, U.N. High Commissioner for Refugees Antonio Guterres called for a massive deployment of international peacekeepers to halt what he described as a "humanitarian catastrophe".

"There is an ethnic-religious cleansing taking place. It must be stopped," Guterres told reporters.

Attackers have reduced mosques to rubble, dragged people from their homes to be lynched and committed massacres.

As many as 100,000 Muslims have fled north, according to Joanne Mariner of rights group Amnesty, redrawing the religious map of the country.

"We are asking to leave this country even if we have to walk. If there is security for a convoy, none of us wants to stay," Mahamate Anker told Reuters at the camp for displaced Muslims in Bangui where she is currently taking refuge.

NO TO PARTITION

U.N. Secretary-General Ban Ki-moon appealed on Tuesday to France to send more troops and warned that the violence meant the de-facto partition of the country was "a distinct risk".

But he said a possible U.N. peacekeeping mission, requested by the country's new Interim President Catherine Samba-Panza, would take time to deploy.

Several Seleka commanders have already mooted the idea of dividing the country. But Samba-Panza told a crowd in the town of Mbaiki, during her first trip outside Bangui as president, that she would not allow the republic to be broken up.

"I have addressed the population to bring to their attention the vague ideas of partition that some are harbouring," she said. "I reaffirm my strong will to not cede a single inch of Central African territory."

French Defence Minister Jean-Yves Le Drian, who met Samba-Panza on Wednesday, said the anti-balaka militia had emerged as the main threat to peace in the country and said those responsible for crimes would be punished.

Human Rights Watch (HRW) said in a report that the anti-balaka, whose name means "anti-machete" in the Sango language, were organised and had started to use language suggesting a desire to eliminate Muslims from the country.

The militia brings together Christians and animists - who wear charms on their bodies for protection - as well as members of the armed forces and supporters of toppled President Francois Bozize.

"Whether the anti-balaka leaders are pursuing a deliberate policy of ethnic cleansing or exacting abusive collective punishment against the Muslim population, the end result is clear: the disappearance of long-standing Muslim communities," said Peter Bouckaert, HRW emergencies coordinator.

In the western gold trading town of Yaloke, fewer than 500 Muslims and one mosque remain out of a previous estimated Muslim population of 30,000 with eight mosques, said Bouckaert.

Muslims used to make up 15 percent of the population of roughly 4 million. Many worked as traders, shopkeepers and herders and their flight is severely affecting the impoverished economy, raising fears of a major food crisis.

The World Food Programme began a month-long emergency airlift of around 1,800 tonnes of food on Wednesday - enough to feed 150,000 people for a month.

International forces including elite units of the Chadian military and African peacekeepers have evacuated tens of thousands of Muslims from areas under the control of anti-balaka forces in the absence of government troops.

The militias are yet to target Muslim populations in the northeast of the country, where Muslims form a majority and some reports say Muslim fighters are regrouping. (Additional reporting by Media Coulibaly; Writing and additional reporting by Daniel Flynn and Matthew Mpoke Bigg; Editing by Joe Bavier and Andrew Roche)