* 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 against Muslims and urged foreign governments to do more to stop the country being torn apart.
Almost one million people, or a quarter of the population, have been displaced by fighting since the mostly Muslim Seleka rebel group seized power in March in the majority Christian country. At least 2,000 people have been killed.
In recent days, Seleka fighters have steadily abandoned the riverside capital Bangui, leaving Muslim civilians at the mercy of Christian militia, known as 'anti-balaka' or 'anti-machete' in the local Sango language.
Looting and mob violence mainly targeting predominantly Muslim neighbourhoods of Bangui have intensified despite the presence of a 1,600-strong French intervention force and thousands of African peacekeepers.
A spokesman for the local Red Cross said it had recovered 13 dead bodies from the streets of Bangui on Monday, in both Christian and Muslim neighbourhoods, and treated eight wounded.
"The security and human rights situation has further deteriorated over the past few days," U.N. High Commissioner for Human Rights Navi Pillay said in a statement.
"Muslim civilians are now extremely vulnerable. Many are being pushed out of the country, alongside ex-Seleka, and are now fleeing, mostly towards the Chadian border," she said.
"We simply cannot let the social fabric of this country be torn apart," Pillay said. "I call as a matter of utmost urgency upon the international community to strengthen peacekeeping efforts... Many lives are at stake."
Bringing some relief to the local population, a World Food Programme convoy of 10 trucks carrying 250 tonnes of cereal arrived in the capital under a French military escort. The convoy had been stranded for three weeks at the Cameroonian border by fears over the security situation.
In a bid to restore a functioning government, new Prime Minister Andre Nzapayeke named a cabinet of 20 ministers including several technocrats, some former ministers and supporters of Seleka and the anti-balaka militia.
Interim President Catherine Samba-Panza, elected by a transitional assembly last week, has pledged to open talks with armed groups and take the country to elections, scheduled to be held by early next year. She has called for more international troops to be deployed.
The European Union last week pledged around 500 additional soldiers, but it was unclear which countries would contribute troops and when they would arrive.
U.N. Secretary-General Ban Ki-moon is due to report to the Security Council next month on options for the likely transformation of the African operation into a U.N. peacekeeping force within six months.
The Catholic Archbishop of Bangui, Dieudonné Nzapalainga, and the head of the country's Islamic community, Imam Oumar Kobine Layama, during a visit to Britain, urged London to back the EU force and plans for a larger U.N. peacekeeping mission.
Highlighting the Muslim plight in Bangui, a Reuters witness in the Miskine neighbourhood watched on Monday as looters dismantled Muslim-owned shops and homes, most of which had already been stripped of their goods.
Gunfire erupted as French forces, firing tear gas, tried to disperse angry Christian crowds attempting to erect barricades on the streets. The witness saw one woman with a gunshot wound.
"As Seleka loses ground, Muslim communities are being systematically attacked by anti-balaka. There's killing on a daily basis," Peter Bouckaert, emergencies director for Human Rights Watch, told Reuters. (Additional reporting by Tom Miles in Geneva and Michael Collett-White in London; Additional reporting and writing by Joe Bavier; Editing by Gareth Jones and Alister Doyle)