News of the capture of Zalingi, which could not be independently verified, broke as RSF and army representatives were engaged in talks sponsored by Saudi Arabia and the United States to reach a truce that would allow humanitarian aid to reach millions of Sudanese affected by the six-month-old conflict.
The fall of Zalingi, in East Darfur State, followed last week's takeover of Nyala, Sudan's second largest city after the capital Khartoum which is virtually controlled by the RSF.
In a statement announcing Zalingi's capture, the RSF said its men have taken prisoner the commander of the local garrison housing the army's 21st Infantry Division, along with 50 other officers and hundreds of troops.
The army had no immediate comment on the claim.
A video shared online by the RSF showed a local paramilitary commander purportedly inside the garrison announcing its fall in the early hours of Tuesday.
The latest win by the RSF gives it control over an oilfield and an associated airstrip which has recently been also used by the army's jet fighters to bomb the paramilitary's positions.
The oilfield and the strip are located across the nearby state border of West Kordofan.
Workers at the oilfield, which produces 10,000 to 12,000 barrels per day, said they were evacuated on Sunday night due to the attack on nearby Zalingi, but an RSF statement on Tuesday sought to reassure them of their safety.
“We would like to assure the company operating the Belila oilfield that our forces … will have nothing to do with the oilfield or the airport and that employees should continue to work for the benefit of the Sudanese people,” the statement read.
The war in Sudan broke out over differences in the implementation of the democratic transition between army chief Gen Abdel Fattah Al Burhan and RSF commander Gen Mohamed Dagalo which boiled over to become a struggle for military and political supremacy.
The conflict has to date displaced nearly six million of Sudan's estimated 48 million population, killed thousands and destroyed much of the infrastructure in the vast and impoverished Afro-Arab nation. It has also plunged in to deep uncertainty the democratic transition the nation embarked on after the ouster of dictator Omar Al Bashir's 29-year regime in 2019.
Past mediations by Saudi Arabia and the United States had produced a series of ceasefires, all of which collapsed or were not diligently observed. This prompted the US and Saudi Arabia to suspend the process this summer. The negotiations resumed last week in Jeddah, but are yet to produce tangible results.
The army and the RSF have been mainly battling each other in Khartoum, but the conflict later spread to the west, firstly to west Darfur where the paramilitary and its local allies killed hundreds in ethnically-driven attacks that forced thousands to flee to neighbouring Chad.
Fighting between the two sides is now under way in other parts of western Sudan, including in and around the cities of El Obeid and El Fasher.
The RSF's forerunner is the notorious Darfur-based Janjaweed militia, which is accused by the International Criminal Court of committing war crimes during the civil war in the 2000s. Back then it fought on the government's side against ethnic African rebels demanding an end to the monopoly on power by the Muslim and Arabised north.
The ICC is investigating the ethnically-driven killings by the RSF and its allies in Darfur which took place this summer.