The latest fighting has hardened battle lines between the two forces in a conflict that threatens to splinter Sudan after plunging half the population into hunger and displacing more than one-fifth since April 2023.
This week, as it attempts to consolidate its territory, the RSF has staged multiple attacks on Zamzam residents, according to three people at the camp.
Medical aid agency MSF has confirmed seven deaths from the violence, while residents say dozens may have been killed. Medics are unable to perform surgery inside Zamzam, and travel to al-Fashir's Saudi hospital, a frequent RSF target, has become impossible, MSF said.
Reuters verified a video showing RSF forces inside Zamzam earlier this week, stamping on a rival flag as a building burned in the background.
Zamzam is located near al-Fashir, capital of North Darfur and the army's last remaining foothold in the wider Darfur region.
The RSF, which did not immediately respond to a request for comment, says Zamzam is a base for the Joint Forces, former rebel groups now fighting alongside the army.
The Joint Forces said in a statement on Thursday that they were not present in the camp. The Sudanese government said the army, Joint Forces, and other volunteers were able to push the RSF back from Zamzam on Wednesday.
Almost 22 months after war erupted from a power struggle between the two factions, the RSF controls most of Darfur in Sudan's west, and much of the neighbouring Kordofan region. The army controls Sudan's north and east and has recently made major gains in Khartoum.
Next week, a "political charter" setting up a parallel government in RSF-controlled territories will be signed, with the announcement of a cabinet coming soon after, Ibrahim al-Mirghani, a politician who supports the effort, told Reuters.
The RSF has targeted Zamzam with artillery for months, causing some people to dig holes for shelter, according to one resident and a video shared by activists.
"Inside the neighbourhoods, they terrorise, steal, and kill ... people hide in these holes when they are firing and when they are raiding, because there is nowhere else to flee," the resident told Reuters.
The RSF has also continued raids and arson attacks on villages surrounding al-Fashir in recent weeks, according to the Yale School of Public Health's Humanitarian Research Lab.
The Yale Lab found that moe than half the structures in Zamzam's main market were destroyed in a manner consistent with arson attacks, executive director Nathaniel Raymond told Reuters.
A video shared by army-aligned Darfur governor Minni Minnawi showed stalls burned to ash and vegetables strewn on the ground.
Arson was also detected on residences along the northern entrance to the camp, said Raymond.
Tens of thousands have been displaced, many seeking refuge in Zamzam and increasing the camp's population to up to one million people, according to the International Organisation for Migration.
Sudan's top UN official Clementine Nkweta-Salami said on Thursday she was "shocked by the attacks on Zamzam IDP camp and the blockages of escape routes."
Across Darfur RSF forces have restricted aid efforts, now also hit by freezes on USAID, according to UN and other aid workers.
MSF, one of few humanitarian groups operating in the area, had to stop a nutrition programme for 6000 malnourished children after the attacks, the aid group's North Darfur co-ordinator Marion Ramstein said.
A global hunger monitor determined in August that Zamzam was experiencing famine. In December, it confirmed famine in two other camps in al-Fashir.
Earlier this month, MSF said it found that the proportion of the camp's children who were malnourished had risen to 34 per cent, a similar level to Tawila, a nearby town to which many have fled from RSF attacks.
