Fears of renewed ethnic slaughter in the Sudanese region of Darfur, where genocidal violence killed as many as 300,000 people two decades ago, have soared in recent days, with a looming assault on an embattled city that is already threatened by famine.

The contest for control of El Fasher, the last city held by Sudan’s military in Darfur, has prompted alarmed warnings from American and United Nations officials who fear that mass bloodshed may be imminent. Linda Thomas-Greenfield, the U.S. envoy to the United Nations, told reporters on Monday that the city was “on the precipice of a large-scale massacre.”

El Fasher is the latest flashpoint in a year-old civil war between Sudan’s military and the Rapid Support Forces, a powerful paramilitary group that the military once nurtured and is now its bitter rival for power. The conflict has devastated one of Africa’s largest countries and created a vast humanitarian crisis that U.N. officials say is one of the biggest in decades.

The crisis also brings a sharp focus on the role of foreign powers accused of fueling the fight, especially the United Arab Emirates.

Since April 14, fighters loyal to the Rapid Support Forces, or R.S.F., have surrounded El Fasher in preparation for what the U.N. has called an “imminent assault.” El Fasher, the former capital of the precolonial kingdom of Darfur, has about 1.8 million inhabitants, including hundreds of thousands who fled earlier waves of fighting.

The city is the last obstacle to total R.S.F. domination of the region. Its fighters swept across Darfur last fall and now hold four of the region’s five major cities.

Control of El Fasher would give the group a block of territory that, combined with neighboring areas, covers about one-third of Sudan and would likely precipitate a shift in the course of the war. One feared scenario is that Sudan splits into rival fiefs as Libya did after the death of Col. Muammar el-Qaddafi in 2011.

At least 43 people have been killed in El Fasher in recent weeks, including women and children, according to the United Nations, in skirmishes and bombings on the edge of the city that residents fear is just a taste of the violence to come.

“Everyone is expecting an attack at any moment,” Dawalbait Mohamed, an El Fasher resident who fled the city last year, and said he was in constant touch with his parents and siblings left behind. “It seems inevitable.”

In the early 2000s, when ethnic slaughter in Darfur was the focus of global attention, the worst atrocities were committed by the Janjaweed — a fearsome group of ethnic Arab fighters that later evolved into the Rapid Support Forces.

Before Sudan plunged into war, R.S.F. leaders had tried to shed their reputations for ruthlessness — although it returned in the past year, amid reports of massacres and looting.

Still, an assault on El Fasher would be risky for the Rapid Support Forces, and potentially costly, experts say. That gives hope to many Western and Arab officials, including some from the United States, that international pressure can still persuade both sides to back down and avert a calamity.

The United Nations Security Council held an emergency session on Monday to discuss the crisis behind closed doors.

After the session, Ms. Thomas-Greenfield said the United States was appealing to all countries — including the United Arab Emirates — to stop support for Sudan’s warring parties, warning that a “crisis of epic proportions is brewing.”

“As I’ve said before, history is repeating itself in Darfur in the worst possible way,” Ms. Thomas-Greenfield said.

Sudan and some U.N. official say the Emirates has supplied the group with money and weapons; The New York Times reported last year on an Emirati weapons smuggling operation to the R.S.F. via eastern Chad.

The U.A.E. has denied any support to the Rapid Support Forces, mostly recently in a letter to the Security Council.

Sudan’s war, which passed the one-year mark on April 15, is escalating and expanding with dizzying speed.

A conflict that began as a power struggle between rival generals — the army chief, Gen. Abdel Fattah al-Burhan and the R.S.F. leader, Lt. Gen. Mohamed Hamdan — has devolved into a sprawling conflict that has drawn in ethnic, religious and rebel groups, on both sides, as well as an array of foreign sponsors.

On Monday, the Russian deputy foreign minister, Mikhail Bogdanov, was in Port Sudan for meetings with Sudanese military and civilian leaders. Russia’s Wagner group supplied missiles to the R.S.F. in the early weeks of the war. The Kremlin has long coveted access to the Red Sea in Sudan.

Elsewhere in Darfur, R.S.F. advances have been accompanied by widespread ethnic violence. U.N. investigators estimated that between 10,000 and 15,000 civilians were killed during an assault on the city of El Geneina, in west Darfur, last October. Most of the victims were from ethnic African groups long targeted by the Arab-dominated Rapid Support Forces. The total number of dead in the yearlong civil war is unknown.

Peace was holding in El Fasher, however, thanks to a local truce between the R.S.F. and other armed groups that surround the city. But that fragile deal crumbled in recent weeks as the Sudanese military persuaded or induced Darfuri groups to abandon their neutral stance, causing the R.S.F. to move in on the city.

The R.S.F. accuses the military of provoking the fight with aerial bombing of R.S.F. controlled areas that, in one case recently, led to the death of seven herders and an estimated 250 camels.

A starving population finds itself caught in the crossfire.

At the Zamzam camp, 10 miles south of El Fasher, 40 percent of children between 6 months and 2 years are severely malnourished, and one child dies every two hours, said Doctors Without Borders in February, calling it an “absolutely catastrophic situation.”

Yet both sides to the conflict are obstructing food aid, according to American and U.N. officials. Sudan’s military has forbidden the United Nations from bringing aid across from Chad except at the sole border crossing controlled by one of its allies.

And the R.S.F. has set up its own controls for foreign aid at Melit, a town just north of El Fasher, bringing deliveries of urgently needed aid to a virtual halt, said a senior U.N. official who could not be identified to avoid compromising aid operations.

Speaking by phone, El Fasher residents worried what would come next.

Shadia Ibrahim, a radio station technician, said she cowered in her home as fierce exchanges of gunfire erupted on Sunday east of the city. The electricity was out, and the prices of water and food were soaring, she said.

Ms. Ibrahim hoped the city would be spared the fate of Geneina, where battle was followed by slaughter. “We hope nothing like that happens here,” she said.