The notorious paramilitary Rapid Support Forces (RSF) have been at the centre of this dispute, which is over the move towards civlian rule.
Headed by Gen Mohamed Hamdan Dagalo - in effect the country's deputy leader - it has been accused of human rights abuses. These include the June 2019 killing of at least 120 protesters at a sit-in demonstration at Army headquarters.
The RSF was formed in 2013 predominantly with fighters from the notorious Janjaweed militia that brutally fought off rebels in Darfur.
In 2015, some 40,000 of its members joined the Saudi-led military intervention in the war in Yemen.
RSF fighters have also been sent to Libya.
In order to facilitate a proposed transition to civilian rule, there was a plan to start integrating the RSF into the army.
But a dispute between Gen Dagolo and the head of the Army, Gen Abdel Fattah al-Burhan, over the timetable for the change and who would head the new integrated force has held things up.
Those tensions spilled over into the fighting which broke out this morning.
