(AFP) - Heavy fighting again rocked Sudan's capital Sunday as tens of thousands have fled the bloody turmoil and a former prime minister warned of the "nightmare" risk of a descent into full-scale civil war.

Army forces clashed with paramilitaries in Khartoum in deadly hostilities that have entered a third week, with the latest widely breached ceasefire scheduled to formally expire at midnight (2200 GMT).

"There has been very heavy fighting and loud gunfire," a southern Khartoum resident told AFP.

Gun battles were also reported around the army headquarters in central Khartoum, and the Sudanese Armed Forces (SAF) carried out airstrikes in the capital's twin city of Omdurman across the Nile River, according to witnesses.

Central Reserve Police, a paramilitary unit, were being deployed across Khartoum, a city of five million, to "protect citizens' properties" from looting, the force said, confirming an army statement.

The police force said it had arrested 316 "rebels", in reference to paramilitary Rapid Support Forces fighters, but this was not confirmed by the RSF, which had previously warned the police against joining the battles.

Foreign nations have scrambled to evacuate their citizens by air, road and sea since the fighting plunged the country into turmoil on April 15.

A first Red Cross plane brought eight tonnes of humanitarian aid to Port Sudan, from Jordan, including surgical material and medical kits to stabilise 1,500 patients.

Millions of Sudanese have endured crippling shortages of water, food, medicines and other basic supplies, while tens of thousands have fled to neighbouring countries, with more on their way.

The fighting is the culmination of a power struggle between army chief Abdel Fattah al-Burhan and Mohamed Hamdan Daglo, head of the RSF and also known as Hemeti.

Sudan's former prime minister Abdalla Hamdok warned Saturday against the conflict's deteriorating into one of the world's worst civil wars.

"God forbid if Sudan is to reach a point of civil war proper... Syria, Yemen, Libya will be a small play," Hamdok told an event in Nairobi. "I think it would be a nightmare for the world."

- Risk of famine -

The violence has killed at least 528 people and wounded about 4,600, the health ministry said, but those figures are likely to be incomplete, with fighting in 12 out of Sudan's 18 states.

Khartoum authorities on Sunday put civil servants on open-ended leave "due to the security situation".

The UN World Food Programme has warned the unrest could plunge millions more into hunger in a country where 15 million people already need aid to stave off famine.

Only 16 percent of hospitals are functioning in Khartoum, according to the World Health Organization, with many facilities shelled in the fighting.

"The situation cannot be sustained" as medical supplies run short, warned Majzoub Saad Ibrahim, a doctor in Ad Damar, north of Khartoum. "This war is ominous, and we hope it stops," he told AFPTV.

The warring sides have agreed to multiple truces but none has taken hold.

The latest three-day ceasefire was agreed Thursday after mediation led by the United States, Saudi Arabia, the African Union and the United Nations.

An envoy of Burhan's met in Riyadh with the Saudi foreign minister, who called for the restoration of "calm" in Sudan, his ministry said.

- 'Threat to Sudan's unity' -

Sudan was ruled for decades by Islamist-backed strongman Omar al-Bashir, who was ousted by the military in 2019 after mass pro-democracy protests.

Burhan and Daglo seized full control in a coup in 2021, before turning on each other.

About 75,000 have been displaced by the fighting, the UN said. At least 20,000 have escaped to Chad, 4,000 to South Sudan, 3,500 to Ethiopia and 3,000 to the Central African Republic, it said.

The fighting has also triggered a mass exodus of foreigners and international staff.

Saudi Arabia said it had taken over 5,000 people to safety on ships across the Red Sea, and Britain said it had taken out just under 1,900 nationals on 21 flights, following large airlifts by France, Germany and other nations.

Fighting, looting and lawlessness have raged in the Darfur region. At least 96 people were reported killed in El Geneina, West Darfur, the UN said.

Darfur is still scarred by a war that erupted in 2003 when Bashir unleashed the Janjaweed militia, which left at least 300,000 dead and close to 2.5 million displaced, according to UN figures.

Daglo's RSF are descended from the Janjaweed.

The Carnegie Middle East Center suggested Daglo's military chances "will ultimately be based on his forces' ability to overcome the SAF's superior firepower in an urban setting".

It argued that the deteriorating humanitarian situation is to his advantage, because "the longer he can hold his ground in Khartoum, the more likely he will have a leverage at the negotiating table.

"On the other hand, if the SAF pushes him out of the capital, Hemeti can potentially mobilise Arab tribes in Darfur and beyond, which could threaten Sudan's unity."

© Agence France-Presse