(AFP) - Fierce fighting raged in Sudan's capital despite a pause Sunday to help those caught up in the violence, on the second day of battles that left three UN staff among more than 50 civilians killed nationwide.

Doctors say hundreds are injured, while the World Health Organization warned "several of the nine hospitals in Khartoum receiving injured civilians have run out of blood, transfusion equipment, intravenous fluids and other vital supplies".

The World Food Programme said it was suspending operations in the impoverished country after the killing of its three workers.

The battles between the powerful armed forces and the Rapid Support Forces (RSF) sparked an international outcry and regional concern, including border closures by neighbours Egypt and Chad.

Deafening explosions and intense gunfire rattled buildings in the capital Khartoum's densely populated northern and southern suburbs as tanks rumbled on the streets and fighter jets roared overhead, witnesses said.

Fighting continued after nightfall on Sunday, as Sudanese hunkered down in their homes with fears of a prolonged conflict that could plunge the country into deeper chaos, dashing long-held hopes for a transition to civilian-led democracy.

Violence erupted early Saturday following weeks of power struggles between army chief Abdel Fattah al-Burhan and his deputy, Mohamed Hamdan Daglo who heads the heavily armed RSF.

The pro-democracy Central Committee of Sudan Doctors reported 56 civilians killed as well as "tens of deaths" among security forces, and around 600 wounded.

Late Sunday afternoon the army said they had "agreed to a United Nations proposal to open safe passage for humanitarian cases", including the evacuation of wounded, for three hours, which ended at 1700 GMT.

RSF confirmed the measure, though they said it would last four hours, and both sides maintained their right to "respond in the event of transgressions" from the other side.

Despite the pause, heavy gunfire could be heard in central Khartoum near the airport, and dense black smoke billowed from the surrounding area.

"The gunfire and explosions are incessant," Ahmed Hamid, 34, from a northern Khartoum suburb, said earlier.

Ahmed Seif, another Khartoum resident, called the situation very worrying and said: "It doesn't seem like it will calm any time soon."

Daglo's RSF say they have seized the presidential palace, Khartoum airport and other strategic locations, but the army insist they are still in control.

As the fighting continued, the stench of gunpowder wafted through Khartoum's streets, deserted except for soldiers.

Medics pleaded for safe corridors for ambulances and a ceasefire to treat the victims because the streets were too dangerous for transporting casualties to hospital.

- 'Justice without delay' -

Fighting also erupted in the western Darfur region and in the eastern border state of Kassala, where witness Hussein Saleh said the army fired artillery at a paramilitary camp.

The UN said its WFP employees had been killed Saturday in clashes in North Darfur and announced a "temporary halt to all operations in Sudan".

After their deaths as well as those of other civilians, UN Secretary-General Antonio Guterres called for "justice without delay". He had earlier warned that an escalation in the fighting would "further aggravate the already precarious humanitarian situation".

The UN says one-third of Sudan's population needs humanitarian aid.

WFP said an aircraft managed by the organisation "was also significantly damaged" at Khartoum airport.

"We cannot do our lifesaving work if the safety and security of our teams and partners is not guaranteed," WFP head Cindy McCain said.

- 'Catastrophic' -

Created in 2013, the RSF emerged from the Janjaweed militia that then-president Omar al-Bashir unleashed against non-Arab ethnic minorities in Darfur a decade earlier, drawing accusations of war crimes.

The RSF's planned integration into the regular army was a key element of talks to finalise a deal that was hoped to restore Sudan's civilian transition and end the political-economic crisis sparked by the military's 2021 coup by Burhan and Daglo.

Appeals to end the fighting have come from across the region and the globe, including the United States, Britain, China, the European Union and Russia, while Pope Francis said he was following the events "with concern" and urged dialogue.

After a meeting on Sudan, the African Union said a senior official would "immediately" travel there on a ceasefire mission.

The Arab League bloc held an emergency meeting in Cairo.

Sudan's former prime minister Abdalla Hamdok, at a press conference in Abu Dhabi, described the humanitarian situation as "catastrophic", called for a truce and appealed for "a helping hand", particularly from Arab countries.

But the two generals appeared in no mood for talks, having described each other as criminals.

Their October 2021 coup triggered international aid cuts and sparked near-weekly protests met by a deadly crackdown.

Burhan, who rose through the ranks under the three-decade rule of now-jailed Bashir, has said the coup was "necessary" to include more factions in politics.

Daglo later called the coup a "mistake" that failed to bring about change and reinvigorated remnants of Bashir's regime ousted by the army in 2019 following mass protests.

© Agence France-Presse