Once they had massed on the perimeter of Sudan’s Zamzam camp, the Rapid Support Forces began the onslaught – shelling, firing from anti-aircraft guns mounted on pickup trucks and storming into the camp chanting racial slurs as they fired on their victims.

An estimated 700,000 people had sought refuge in Zamzam, Sudan’s largest displacement camp, but last weekend they were forced to seek cover and plot the best escape route. Most had fled these fighters before.

Those who were able gathered any belongings that could be carried on their backs or flung on to donkeys and camels and rushed to begin the long walk to El Fasher city, 14km (8.7 miles) away, or Tawila displacement camp, 60km west of Zamzam.

Mohamed*, a community organiser, tells the Guardian he tried to sneak past the fighters to reach the medical centre staffed by the NGO Relief International that was hit during the early stages of the attack on 11 April, when nine staff were killed, including one of his friends.

“They were barbaric, inhumane. They were chanting as they killed people in their homes. It is behaviour you wouldn’t even find in the wilderness,” he says, adding that the fighters, who claimed to be seeking Sudanese government fighters hiding in the camp, attacked people in their homes or in their cars as they tried to escape.

“I ran into an RSF vehicle – the fighters were shouting racist slurs and started firing at us. I was shot in my right leg, then someone who was hiding in one of the homes dragged me inside.”

Rescuers had only salt and leaves to treat and dress his wound. They spent the next two days in hiding.

The battle for Zamzam raged for three days. The RSF and its allied militias claimed they had seized control of it on 13 April. At least 400 civilians, including women and children, had been killed in Zamzam and nearby Um Kadada by 15 April, according to the UN Office for the Coordination of Humanitarian Affairs, although it says it has not had access to assess the real scale of the damage.

For most people this is not the first time they have escaped from the RSF. The camp grew in size during the current civil war, as people fled other parts of Darfur taken by the RSF, a collection of militia who follow the former warlord Mohamed Hamdan Dagalo, known as Hemedti. But the camp has existed since the 2000s, before the war. The longer-term inhabitants fled similar violence by the RSF in its prior form as the Janjaweed militias.

Another Zamzam resident says he was in his house when the shelling started, causing a fire to break out around him. The neighbours banded together, gathered the elderly people and ran towards the north for the road to El Fasher.

“The shelling was intense. People started running everywhere, to the south, east, west. The shelling was so intense and they were using all types of heavy weaponry, we couldn’t even speak to each other. We walked by foot – it was tiring and difficult. We would take breaks to sit and sometimes people just collapsed on the ground.”

The UN estimates 400,000 people had fled Zamzam by Tuesday, heading either to El Fasher or to Tawila.

Medecins Sans Frontieres’ project coordinator for North Darfur, Marion Ramstein, says 10,000 people arrived in Tawila in the first 48 hours of the onslaught on Zamzam, most in an advanced state of dehydration and exhaustion.

“Some children were literally dying of thirst upon arrival, after travelling for two days under a burning sun, without a single drop of water to drink,” says Ramstein, who says the hospitals are so overcrowded that children are having to share beds.

A displaced person already living in Tawila says he saw thousands of families arrive in Tawila hungry, thirsty and often with injuries after the arduous journey.

“Many of them came on foot. Some of those who had cars were stopped on the way and then looted [by fighters] and many of the youth were disappeared or killed,” he says. “The families here are out in the open without water.”

The situation is similar in El Fasher, where the man who fled his burning home in Zamzam says most of the injured are still waiting to be treated or have been given crude first aid, such as using fire to cauterise their wounds.

A doctor in El Fasher says there is an urgent need for shelter, food and water but the area’s ability to provide them is limited by a year of siege on El Fasher and its vicinity – the last major city in Darfur that the RSF does not yet control after more than two years of war.

“Even now I can hear the rumbling of heavy artillery nearby. The RSF is always bombing somewhere in El Fasher, 24 hours,” he says. “RSF has looted all the outskirts of El Fasher, killing many people, burned a lot of villages, looted their property.”

The siege has meant that prices in the food markets are high. According to a list of market prices distributed by the North Darfur governorate, after the attack on Zamzam the price of a kilogram of wheat rose 3,000 Sudanese pounds (£3.80) to 15,000 when bought with cash but was as high as 22,000 when bought using mobile banking, which most people rely on. Famine had already taken hold in Zamzam camp, and the latest fighting has added to the crisis.

While at least half of Zamzam’s population has fled, a significant number are unable to leave. Mohamed and other campaigners accuse the RSF of holding them hostage and using them as human shields to prevent the Sudanese army from launching a counterattack.

He says fighters are stopping people on the roads and choosing who they allow to pass based on skin colour.

“The main goal is a full-scale mass genocide and to displace any tribe not associated with the RSF,” says Mohamed.

A communications blackout has meant that details of the aftermath of the attack and how many people have been killed and injured cannot be established but information is slowly trickling to families outside Darfur.

Altahir Hashim, a UK-based Darfuri campaigner, says that only after several days did he discover that his mother and siblings were able to escape but that several of his cousins had died. Many of his friends also lost family members.

According to the Yale School of Public Health’s Humanitarian Research Lab, which uses satellite imagery to monitor violence in Sudan, fires continued to spread after the RSF took control of Zamzam, with 1.7 sq km of the camp – equivalent to 24 football pitches – destroyed by fire between 11 and 16 April.

“Even until now the people who are still in the camp are being killed and raped. Even those who tried to escape to the west, they brought some of the little girls back, the elderly and they are killing them. Until now there are many wounded who have not been treated,” he says.

“The people [who escaped] are truly exhausted because what happened in Zamzam is a serious tragedy. They’re indescribable, things that have not happened in humanity before.”

* Name has been changed