* Reuters witness sees at least dozen bodies at military camp
* Red Cross confirms mass grave discovered at camp in Bangui
* Top U.N. official warns of 'ethnic-religious cleansing' (Adds U.N. official, details)
BANGUI, Feb 12 (Reuters) - African Union peacekeepers on Wednesday uncovered a mass grave at a military camp in Bangui occupied by the Muslim Seleka rebels, as a top U.N. official warned Central African Republic was succumbing to "ethnic-religious cleansing".
A Reuters witness saw four separate shallow graves containing at least a dozen bodies at the military camp in the 200 Villas neighbourhood of central Bangui, where some Seleka fighters are still stationed.
Pastor Antoine Mboa Bogo, head of the local Red Cross, confirmed there was a mass grave at the camp but said his staff had not yet had time to determine the number of dead.
"We need to send in a team with the proper equipment tomorrow to examine it," he said.
On a visit to Bangui, U.N. High Commissioner for Refugees Antonio Guterres called for a massive deployment of international peacekeepers to restore stability in a country gripped by violence since March, when Seleka seized power.
Months of looting, rape and murder followed, bringing international pressure that saw Seleka leader Michel Djotodia resign in January. Since then, Christian militia have carried out systematic attacks on Muslims, forcing tens of thousands of people to flee the country.
"We are witnessing a humanitarian catastrophe in Central African Republic," Guterres told reporters. "There is an ethnic-religious cleansing taking place." (Reporting by Serge Leger Kokpakpa; Writing by Daniel Flynn; Editing by Bate Felix)