Sudan's de facto leader, Gen Abdel Fattah al-Burhan, told CNN earlier on Tuesday the ceasefire would start at 16:00 GMT. Some elements of the army have denied this.
RSF head Mohamed Hamdan Dagalo, who is better known as Hemedti and is also Sudan's deputy leader, tweeted that he had approved a ceasefire to ensure the evacuation of wounded civilians, but said previous deals to halt fighting had been violated.
Mr Perthes said agreements to pause the fighting for several hours on Sunday and Monday were not fully observed.