UN leader Ban Ki-moon warned Monday that anarchy in Central African Republic risks spiralling further out of control and could need up to 9,000 peacekeepers.
Ban expressed alarm about increasing retaliatory attacks between Muslim and Christian groups in a report which calls on the UN Security Council to act urgently on the crisis.
Rebels overthrew Central African Republic's president in March but a transitional government has lost all grip on the huge but impoverished country of 4.5 million people.
"The population lives in fear," Ban said in the report, obtained by AFP ahead of a Security Council meeting on the crisis next Monday.
Ban suggests five ways in which the international community can act.
Four involve providing various forms of financing and logistical support for an African force already in the country.
The fifth option is a UN peacekeeping force of between 6,000 and 9,000 troops, plus 1,700 police.
While Ban does not opt for any of the options, the report makes it clear that the African force, officially known as MISCA, even reinforced with international finance will be less effective than UN peacekeeping troops.
"When you read the report with attention, you understand that the African force will not be able to confront the crisis and that only a UN peacekeeping mission can avoid the worst case scenario," said Philippe Bolopion, Human Rights Watch's UN specialist who has just returned from a mission to Central African Republic.
UN diplomats also said the UN leadership prefers a peacekeeping force but that African countries want to handle the conflict. "The first stage is likely to be a special fund for the African force," one diplomat said.
The 2,500 strong African force is eventually to be increased to 3,650 troops.
But violence is already worsening in the resource-rich state where a coalition of rebels, known as Seleka, forced President Francois Bozize to flee in March.
Armed gangs, mainly consisting of former Seleka fighters, now control most of the country. But there has been an increase in clashes between Muslim groups and self-defence units formed by Christian villages.
"The increasing attacks and indiscriminate retaliations have created a climate of deep suspicion between Christians and Muslims in some areas of the country," Ban said.
"Further manipulation of these fears might well lead to uncontrollable sectarian violence with untold consequences," Ban said.
The UN leader said the Security Council should consider "sanctions" against perpetrators of mass rapes and killings already carried out.
"There is an urgent and growing need to address the crisis before it spirals out of control," Ban warned the council.
A failure to act now would mean a future intervention would be "more complex and costly," Ban said.
Highlighting the "high potential of widespread atrocities" in Central African Republic, Ban said the Security Council must act "in a manner commensurate with the complexity of the crisis and the desperate protection needs of the population."
France is expected to propose a resolution in coming weeks on how the African force can be helped, diplomats said. Formal moves to launch a peacekeeping force could be started in 2014, they added.