The recapture of the city of Wad Medani, the capital of Gezira province, came more than a year after it fell to the RSF.
Wad Medani had previously been a safe haven for displaced families in the early months of the war.
The conflict in Sudan started in April 2023 when simmering tensions between the leaders of the military and the RSF exploded into open fighting in the capital, Khartoum, and other cities across the sprawling north-eastern African country.
The conflict has killed more than 28,000 people, forced millions to flee their homes and left some families eating grass in a desperate attempt to survive as famine swept parts of the country.
It has been marked by gross atrocities including ethnically motivated killing and rape, according to the United Nations and rights groups.
The International Criminal Court said it was investigating alleged war crimes and crimes against humanity.
Earlier this month, the Biden administration said that the RSF and its proxies are committing genocide, and slapped sanctions on the RSF leader, Mohamed Hamdan Dagalo, as well as seven RSF-owned companies in the United Arab Emirates, including one handling gold likely smuggled out of Sudan.
Wad Medani had previously been a safe haven for displaced families in the early months of the war. (AP PHOTO)
The military said in a statement that its forces had entered Wad Medani early Saturday and that they were working to "clear the rebels' remnants inside the city".
"Congratulations to the Armed Forces, their supporting forces everywhere and to our people as they reclaim their dignity, security and stability," the statement said.
There was no immediate comment from the RSF.
Culture and Information Minister Kahlid Aleiser, who is the government spokesman, said the military and its allies "liberated" Wad Medani, which is about 100 kilometres southeast of Khartoum.
Soldiers posted videos on social media purportedly showing forces celebrating with residents in the city centre. One video showed residents taking to the streets to celebrate the "liberation" of the city, and shouting, "Allah is Great".
Since the RSF captured Wad Medani in December 2023, tens of thousands of people have been forced to flee their homes in and around the city.
In recent months the RSF has suffered multiple battlefield blows, giving the military the upper hand in the war.
It has lost control of many areas in Khartoum, the capital's sister city of Omdurman, and the eastern and central provinces.
The war has created the world's largest displacement crisis, driving over 14 million people — about 30 per cent of the population — from their homes, according to the United Nations.
An estimated 3.2 million people have crossed into neighbouring countries including Chad, Egypt and South Sudan.
Famine has been also detected in at least five areas, including three camps for displaced people in the western Sudanese region of Darfur, according to an international monitoring project, the Integrated Food Security Classification.
It said five other areas are projected to experience famine in the next six months. More areas are also at risk of famine, it said.
