* Mainly Muslim Seleka rebels losing ground
* Muslim civilians threatened by Christian militias
* Senior U.N. official urges more support for peacekeepers (Adds Red Cross on deaths, WFP convoy arriving)
By Emmanuel Braun
BANGUI, Jan 27 (Reuters) - At least 13 people were killed in Central African Republic on Monday as the top U.N. human rights official warned of escalating reprisals ...