CAIRO -- Clashes resumed between Sudan's military and a powerful paramilitary force after a three-day cease-fire expired Wednesday morning, a protest group and residents reported.

Sudan descended into conflict in mid-April after months of worsening tensions exploded into open fighting between rival generals seeking to control the African nation. The war pits the military, led by Gen. Abdel-Fattah Burhan, against the Rapid Support Forces, a militia-turned-paramilitary force commanded by Gen. Mohammed Hamdan Dagalo.

The cease-fire, brokered by the United States and Saudi Arabia, expired Wednesday at 6 a.m. local time. The truce had brought relative calm to Sudan's capital, Khartoum, since it took effect, but fierce fighting was reported starting Tuesday night in parts of the city.

Residents said the clashes centred around an intelligence headquarters near Khartoum International Airport. There were sporadic clashes elsewhere in the capital, according to three people who live in the capital.

"The battles have been intensified," said Khalid Abdel-Rahman who lives in Khartoum's city centre. "Sounds of gunfire echoed across the area."

Fierce clashes were also reported around a military facility in the neighbouring city of Omdurman, according to area resistance committees that are part of a wider group that spearheaded pro-democracy demonstrations over the past years.

The resumption of fighting signals that U.S. and Saudi efforts to extend the truce have failed. Both Washington and Riyadh have been mediating between the warring factions to stop the clashes.

The conflict has been centred largely in the capital and western Sudan's Darfur region, which have seen ethnicity-motivated attacks on non-Arab communities by the Rapid Support Forces and allied militias, according to UN officials.

The fighting has killed thousands of people and forced more than 2.5 million people to flee their homes to safer areas in Sudan and neighbouring countries, according the UN migration agency.

The UN said Wednesday that within the last month, it has helped truck 17 tons (15.4 metric tons) of aid to various parts of Sudan, including 50 truckloads in the first two days of the latest cease-fire.

"We will continue to deliver, cease-fire or not," UN spokesperson Farhan Haq said at the world body's headquarters, while emphasizing that the fighting needs to stop so that the world body can reach all people in need.

438 trucks with 17K tons aid (50 of those trucks moved during first two days of the most recent cease fire).