The gunmen arrived at dawn on motorcycles, horses and in cars. For hours afterward, they fired into houses, rampaged through shops and razed clinics, witnesses said, in a frenzied attack that upended life in El Geneina, a city in the Darfur region of Sudan.

The violence in mid-May, which killed at least 280 people in two days, came just hours after two military factions that have been battling for control of Sudan signed a commitment to protect civilians and allow the flow of humanitarian aid.

Truce agreements have so far failed to end the brutal fighting that broke out on April 15 between the Sudanese army and its rival, the paramilitary Rapid Support Forces. Peace talks in Saudi Arabia were formally suspended last Thursday.

The fighting has decimated many areas of the capital, Khartoum. But the war between the military factions has also swept across the country to the long-suffering western region of Darfur — an area already blighted by two decades of genocidal violence.

The gunmen who poured into El Geneina were backed by the paramilitary forces. They were met with fierce resistance from armed fighters, including some of the city’s residents, who had received weapons from the army, according to doctors, aid workers and analysts.

Amid the fighting, scores of markets were destroyed, dozens of aid camps burned and health facilities were shuttered. As heavy artillery rained from the sky, militants went door-to-door to find targets and shoot at unarmed civilians. With no food or water amid the 100-degree heat, thousands began fleeing the city — only to be killed by snipers, leaving bodies piled in the streets.

“The situation is catastrophic in parts of Darfur,” said Toby Harward, the coordinator in Darfur for the United Nations refugee agency who has been receiving the displaced in the neighboring country of Chad. “Its people are living in a dystopian nightmare where there is no law and order.”

Communications to West Darfur have been cut off for two weeks. But interviews over the last week with two dozen displaced people, humanitarian workers, United Nations officials and analysts revealed that the region is besieged by levels of violence unlike any in recent years. More than 370,000 people have fled Darfur in the past seven weeks, according to the International Organization for Migration.

Many of those displaced are reaching border towns like Adré in Chad, hungry and traumatized, narrating harrowing stories about their escape.

They include Hamza Abubakar, a 30-year-old who fled the village of Misteri in West Darfur after it was attacked at dawn in late May by Arab militants backed by the Rapid Support Forces. As people fled their homes, he said, the militants, who wielded AK-47s and other guns, chased them on horses, camels and in cars. Mr. Abubakar had a bullet wound in his left arm and was recuperating at a clinic.

“They had no reason to start killing us,” Mr. Abubakar said in a phone interview. Even though his wife and 1-year-old daughter made it out, he said, his brother and sister had died in the street from their injuries.

“Many others could not make the journey,” he said.

For years, the government of the former dictator Omar Hassan al-Bashir waged a campaign of murder, rape and ethnic cleansing in Darfur that killed as many as 300,000 people since 2003.

The two generals now vying for power in Sudan — Gen. Abdel Fattah al-Burhan of the army and Lt. Gen. Mohamed Hamdan of the paramilitary forces — were among those who perpetrated those atrocities, which eventually led to an indictment of Mr. al-Bashir at the International Criminal Court.

Fighting in the region has also surged in recent years after U.N. peacekeepers departed and mercenaries and rebel fighters flooded through porous borders with neighboring Libya and Chad. African farmers and nomadic Arab herders — at times backed by General Hamdan’s men — also clashed over dwindling resources and land.

In the weeks before the war started, tensions were already rising in Darfur.

In various cities across the region, community leaders, aid workers and observers reported a weapons buildup and increased recruitment campaigns by both the army and the paramilitary forces. General Hamdan, whose forces are mainly recruited from Arab tribes, also began enlisting soldiers from African tribes in a bid to curry favor with them and bolster his power in the region.

When the fighting began in Khartoum in April, the rival military forces also began clashing in Darfur, leading to mass killings of civilians, looting of food warehouses and attacks on aid workers.

But community leaders, civil society organizations and some regional political leaders were able to quickly negotiate a truce that halted the fighting in parts of Darfur. A truce in East Darfur has largely held, observers said, even though insecurity persists because of attacks by bandits.

That opened a small window of opportunity that allowed U.N. staff and international humanitarian workers across Darfur to be evacuated in late April by road and by air to Chad and South Sudan.

But shortly after the evacuations, the region descended into chaos yet again.

The two sides began clashing over control of key installations, including the airport and military bases in cities such as El Fasher in North Darfur and Zalingei in Central Darfur. In the city of Nyala in South Darfur, clashes ensued and banks were looted after paramilitary members were unable to collect their salaries because General al-Burhan had frozen their accounts and assets, aid workers and analysts said.

Arab militants backed by the paramilitary forces also mobilized and advanced toward El Geneina, where the army was already arming members of ethnic African tribes to defend themselves.

“El Geneina is one of the worst places to be on Earth at this moment,” said Fleur Pialoux, a project coordinator for Doctors Without Borders in El Geneina, who evacuated the city in late April.

Before the conflict, her team had been racing to combat a wave of malaria and malnutrition in Darfur ahead of the June rainy season.

But as bullets riddled her staff’s compound, Ms. Pialoux, 30, knew she had to get her workers out. After four days of huddling in a safe room and scouring social media apps for news of a cease-fire, she learned of a brief truce to allow for bodies to be collected from the streets. As she and her staff fled the city, Ms. Pialoux recalled speeding past scorched displacement camps, a looted market and razed roads.

The warring parties in Darfur, she said, “will stop at nothing until they run out of ammunition or bodies to kill.”

With the collapse in the cease-fire talks in Saudi Arabia and the call to arms issued by the governor of Darfur, Mini Arko Minawi, the region could be drawn into more vicious and protracted warfare.

Aid workers are unable to obtain visas to get into Sudan or find safe routes to deliver food by road. The prices of food, water and fuel have skyrocketed, and many people are unable to access cash.

On Monday, the army was accused by the government of the Democratic Republic of Congo of bombarding a university in Khartoum on Sunday, killing 10 Congolese citizens. A spokesman for the army did not reply to an immediate request for comment.

In El Geneina, one Sudanese doctor who had been sheltering with a colleague in a medical guesthouse in late April said armed gunmen beat and robbed them before depositing them in the streets.

“The roads were filled with the smell of death and gunfire,” said the doctor, 30, who asked to be called by his nickname, Yousef, for security concerns. “Bodies were decomposing in the streets, covered in bullet wounds.”

He and his colleague lived on the run for the next month, he said, dodging gunfire and roving militias on motorbikes to reach a string of temporary shelters: a mosque, an abandoned clinic, a scorched market.

“The city was flooded with guns of all types. I have never seen anything like this” said the doctor, who had worked in El Geneina for four years. He said that he witnessed gunmen kill residents indiscriminately, and when armed groups started going door to door in late May, killing residents, he and his colleague fled.

At least a dozen women have been raped in El Geneina, according to Mona Ahmed, a women’s rights activist who fled the city last month. Ms. Ahmed said the real number of rape victims is most likely higher.

“There is no protection for them, no medical or social support,” Ms. Ahmed, 27, said. “Terror thrives in that kind of environment that is cut out from the rest of the world.”

Elian Peltier contributed reporting from Chad.