The fighting pits Sudan's army against the powerful paramilitary Rapid Support Forces (RSF) and has escalated a humanitarian crisis, forcing over 1.3 million people to flee their homes and threatening to destabilize the wider region.

The cease-fire brought a relative lull in fighting in Khartoum on Tuesday, but little sign of a rapid scale-up in humanitarian relief.

Witnesses reported clashes in several areas of the capital on Wednesday afternoon, and Saudi Arabia and the United States, as well as the warring parties, are monitoring the cease-fire.

The conflict in Sudan erupted as plans for an internationally backed political transition toward elections under a civilian government were set to be finalized, bringing sustained air strikes and ground fighting to the capital for the first time.

The United Nations human rights chief called the situation in Sudan "heartbreaking" and said there were "very deeply troubling" accounts of sexual violence in Khartoum and Darfur with at least 25 cases reported so far and the real number likely much higher.

Aid workers said many of the supplies and staff arriving at Port Sudan on the Red Sea coast have been awaiting security permits and guarantees.

The UN reports that the number of people in need of aid within Sudan has increased to 25 million, or over half the population.

However, donor contributions to the refugee response plan are scarce, and more resources are needed urgently to support the countries hosting refugees.