The UN has found "ample evidence" of war crimes committed by both sides in the religious conflict that has devastated the Central African Republic (CAR), but is downplaying earlier claims that the country is in the throes of ethnic cleansing and genocide.

Violence has engulfed CAR since March 2013 when the mainly Muslim rebels of the Seleka alliance seized the capital, Bangui, and installed the country's first Muslim president, Michel Djotodia.

The Seleka went on to terrorise the majority Christian population, killing men, women and children until they were forced from power five months ago. Predominantly Christian forces known as the anti-balaka (anti-machete) militia retaliated by slaughtering thousands of Muslims and forcing hundreds of thousands more from their homes.

In February, the UN high commissioner for refugees, António Guterres, said CAR was experiencing "massive ethnic-religious cleansing", while Amnesty International has warned of a "Muslim exodus of historic proportions". But in a report obtained by the Associated Press (AP) on Thursday, the UN's assessment is less bleak, describing talk of ethnic cleansing and genocide as premature.

"Ample evidence exists to prove that individuals from both sides of the conflict perpetuated serious breaches of international humanitarian law and crimes against humanity as well as war crimes," it says.

"[But] the fact that there is an anti-Muslim propaganda from some non-Muslim quarters does not mean that genocide is being planned or that there is any conspiracy to commit genocide or even a specific intent to commit genocide." It adds that the displacement of Muslims is a matter of "protection and the preservation of human life, not a matter of ethnic cleansing".

However, the report counsels against complacency: "If the international community does not react with speed and determination by sending more peace keeping forces to CAR, we may soon face a situation which will rapidly deteriorate and bring about genocide and ethnic cleansing."

Its conclusions drew a swift and angry response from Amnesty International, which accused the UN of failing to grasp the reality of the crisis. "I would say that … the report is ignoring the fact that the massive displacement of the Muslim population in the Central African Republic is not simply a consequence of the violence there, but its goal," the senior crisis response adviser, Joanne Mariner, wrote in an email to AP. She added that Christian militia fighters "have made no secret of their intent to kill or forcibly expel all Muslims from the areas under their control".

The UN report also accuses neighbouring Chad and Sudan of contributing to the violence, pointing out that Seleka forces included mercenaries from both countries and saying "there is enough evidence" to prove that Chad provided the rebels with military and financial support.

Chad, which is predominantly Muslim, withdrew its soldiers from the African Union-led peacekeeping force in April following accusations that they were siding with the Seleka rebels. The country's foreign ministry said Chad had been subjected to a "gratuitous and malicious campaign" despite its sacrifices in the CAR.

On Wednesday, Guterres and Ertharin Cousin, the executive director of the UN World Food Programme, urged international donors to support their agencies' efforts to deal with the refugee crisis.

According to the UN, almost 90,000 people – most of them women and children – have fled across the border between CAR and Cameroon since December 2013. It puts the acute malnutrition rates among the refugees at 20-30% – well above the emergency threshold of 15%.

Cousin said: "Women and children are arriving in Cameroon in a shocking state, after weeks, sometimes months, on the road, foraging for food. After they finally escape the danger they faced in CAR, children and women who flee must not die from malnutrition."

Earlier this week, Human Rights Watch (HRW) warned that conditions in the towns where thousands of Muslims are sheltering are so dire that the government and international peacekeepers should consider allowing them to seek sanctuary in neighbouring countries.

The NGO said that most of the Muslims still in the western part of the CAR are ethnic Peuhl nomads, who are living in small and increasingly dangerous enclaves guarded by African Union peacekeepers and French troops.

Militia attacks have led the foreign soldiers guarding them to impose severe restrictions on the movements of those in such towns as Boda, Carnot and Yaloké. Faced with unsanitary and cramped conditions and often lacking proper medical care, many Peuhl want to flee CAR.

HRW's emergencies director, Peter Bouckaert, said: "Conceding that many threatened Muslims in the Central African Republic need to leave the country temporarily is an option of last resort, but there are no other immediate solutions. Many Muslims in western Central African Republic have clearly and unequivocally expressed a desire to flee to neighbouring countries, and they should be allowed to leave safely."