UNITED NATIONS, Apr. 29 (Reuters) - The U.S. is appealing to all countries - including the United Arab Emirates - to stop support for Sudan's warring parties, the U.S. envoy to the United Nations said on Monday, warning that a "crisis of epic proportions is brewing."

War erupted in Sudan one year ago between the Sudanese army (SAF) and paramilitary Rapid Support Forces (RSF), creating the world's largest displacement crisis. The U.N. has voiced concern in recent days about a possible imminent RSF attack on al-Fashir in Sudan's North Dafur region.

The fight for al-Fashir, a historic center of power, could grow more protracted, inflame ethnic tensions that surfaced in the region 20 years ago and reach across Sudan's border with Chad, say residents, aid agencies and analysts.

"As I've said before, history is repeating itself in Darfur in the worst possible way," U.S. Ambassador to the U.N. Linda Thomas-Greenfield told reporters on Monday, adding that al-Fashir was "on the precipice of a large-scale massacre."

In the early 2000s the U.N. estimates some 300,000 people were killed in Darfur when "Janjaweed" militias - from which the RSF formed - helped the army crush a rebellion by mainly non-Arab groups. Sudanese leaders are wanted by the International Criminal Court for genocide and crimes against humanity.

Top U.N. officials warned the Security Council this month that some 800,000 people in al-Fashir were in "extreme and immediate danger" as violence worsens and threatens to "unleash bloody intercommunal strife throughout Darfur."

Al-Fashir is the last major city in the vast, western Darfur region not under RSF control. The RSF and its allies swept through four other Darfur state capitals last year, and were blamed for a campaign of ethnically driven killings against non-Arab groups and other abuses in West Darfur.

"We do know that both sides are receiving support - both with weapons and other support - to fuel their efforts to continue to destroy Sudan and yes, we have engaged with the parties on that including with our colleagues from the UAE," Thomas-Greenfield said.

U.N. sanctions monitors have described as "credible" accusations that the United Arab Emirates had provided military support to the RSF. The UAE has denied involvement in military support to any of Sudan's rival parties.

"The United Arab Emirates ... is not supplying any arms or ammunition to any faction engaged in the ongoing conflict in Sudan," UAE Ambassador to the U.N. Mohamed Abushahab wrote to the Security Council on April 25.

He added the UAE "categorically rejects any insinuation that it has extended financial, logistical, military assistance, or diplomatic support to any armed group in Sudan."

The U.N. has said nearly 25 million people, half of Sudan's population, need aid and some 8 million have fled their homes.

A U.N.-backed global authority on food security has called for immediate action to "prevent widespread death and total collapse of livelihoods and avert a catastrophic hunger crisis in Sudan."

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