Sudan's army and the paramilitary Rapid Support Forces are locked in heavy clashes in the south-western region of Kordofan and localities south and west of Omdurman, part of the capital's greater region, residents said on Wednesday.
The army and allied militias, they said, regained control on Tuesday night of the town of Al Khawai in western Kordofan, after the RSF captured it a week earlier.
Ahmed Hussein Mustafa, a spokesman for a joint army-led force, claimed they ambushed a large contingent of RSF fighters on Tuesday, killing about 800 of them and seizing vehicles, weapons and ammunition.
There was no immediate response from the RSF on Mr Mustafa's claim, but images shared online on Wednesday purported to show army troops and allied militiamen posing outside the town's main police station.
The authenticity of the images could not immediately be confirmed by The National.
A medical war monitoring group on Kordofan said that four civilians were killed and eight injured in what it described as random shelling by the RSF on the army-held city of El Obeid.
The claim by the group could not be immediately confirmed.
The army-led contingent marched on to Kordofan from central Sudan and is believed to be on the way to attempt to regain control of the RSF-held city of En Nahud before marching on to Darfur.
The RSF is in virtually total control of Darfur, with El Fasher being the only city still under army control. The RSF has been besieging the city since May last year, sparking heavy fighting that has sent hundreds of thousands of people fleeing to other parts of Darfur and created a steep humanitarian crisis.
The fighting in Kordofan, a vast region that lies in south-western Sudan, comes nearly two months after the army and its allies expelled the RSF from the Sudanese capital, which had been under the paramilitary's control since the early days of the country's two-year civil war.
The war has claimed tens of thousands of lives, displaced more than 13 million and left some 26 million facing acute hunger, with pockets of famine, many in Darfur, surfacing in several parts of the vast Afro-Arab nation of 50 million.
In the outlying areas of Omdurman, heavy fighting has been raging for nearly a week between the RSF and the army, with the latter making modest gains in the face of stiff RSF resistance, according to witnesses and war-monitoring groups in the area.
They said the army and its allies on Tuesday captured two Omdurman districts – Al Jamia and Al Shaqla. The area is home to the campus of the Islamic University, where high rises had been used by RSF snipers to slow the advance of the troops and allied militiamen, they added.
Al Shafie Ahmed reported from Kampala, Uganda.
