Sporadic clashes between the Sudanese army and a powerful paramilitary force spilled over into Thursday, puncturing the relative calm in the capital of Khartoum and raising the risk that a week-long internationally-brokered truce would crumble.
The cease-fire, which is being monitored by Saudi Arabia and the United States as well as the warring parties, was reached after five weeks of warfare in Khartoum and outbursts of violence in other parts of Sudan, including the western region of Darfur.
The fighting pits Sudan's army against the paramilitary Rapid Support Forces (RSF) and has worsened a humanitarian crisis, forced over 1.3 million people to flee their homes and threatened to destabilize the wider region.
The army, led by career officer General Abdel Fattah al-Burhan, relies on airpower while the RSF, commanded by former militia leader General Mohamed Hamdan Dagalo, widely known as Hemedti, has spread out and taken cover in Khartoum's streets.
It is unclear whether either side has gained an edge in a conflict that threatens to create a large-scale humanitarian crisis and destabilize regional countries.
Clashes between rival military factions broke out on Wednesday in Khartoum and other cities, residents said.
The city of Zalingei, capital of Central Darfur State, has been under siege by armed militia for the past few days, and the U.N. refugee agency's Darfur coordinator Toby Harward called on authorities to regain control of the city.
Telecommunications have been cut off, and gangs roaming the city on motorcycles have attacked hospitals, government and aid offices, banks, and homes.
A similar situation has been reported in the West Darfur State capital El Geneina, which has also been subject to a telecommunication blackout for several days after hundreds were killed in militia attacks.
The cease-fire was agreed to on Saturday following talks in Jeddah mediated by Saudi Arabia and the United States. Previous cease-fire announcements have failed to stop the fighting. In statements late on Wednesday, the army and RSF accused each other of violating the agreement.
The RSF said it was forced to defend itself against land, artillery and air strikes by the army. The army in turn accused the RSF of attacks on the country's mint, army airbases and several cities west of the capital.
Sudan's armed forces issued a statement accusing the RSF of breaching the cease-fire. They claimed to have repelled the attacks and destroyed six enemy vehicles.
The conflict erupted in mid-April as plans for an internationally backed political transition toward elections under a civilian government were set to be finalized.
U.N. humanitarian agency OCHA said agencies were ready to deliver aid to more than 4 million people, but that bureaucratic blockages as well as security issues were hampering distribution.
Out of the 168 trucks ready to deliver assistance, just a small number were on the move from Port Sudan to Gadaref, Kassala and Al Gezira, an aid official told Reuters.
Many residents are struggling to survive as they face prolonged water and power cuts, a collapse of health services and widespread lawlessness and looting.
Sudan was facing severe humanitarian pressures even before the conflict broke out.
More than 1 million people have been displaced within Sudan and 319,000 have fled to neighboring countries, some of which are similarly impoverished and have a history of internal conflict, according to the International Organization for Migration.
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Many have crossed into Chad and Egypt in the last few days, said Filippo Grandi, head of the U.N. refugee agency.