Heavy bombardments could be heard in east Khartoum, witnesses said, and one resident shared a picture of thick black smoke rising into the sky.
In Omdurman and Bahri, Khartoum's twin cities, people said they heard sounds of weapons firing.
After five weeks of fierce battles between the army and the paramilitary Rapid Support Forces, the warring factions on Saturday agreed to a seven-day truce starting at 9.45pm on Monday, which was intended to allow for the delivery of aid.
It would be the latest in a series of ceasefires, all unsuccessful, declared since the fighting began on April 15.
Each side has blamed the other for breaches of the agreements.
The latest truce, according to a joint Saudi-US statement, could be extended, subject to the agreement of both sides.
The Sudanese Armed Forces and the RSF said they would honour the truce.
In a statement carried by the official Saudi Press Agency, Riyadh acknowledged the many broken ceasefires in Sudan since fighting began.
But the Saudi Foreign Ministry said that “unlike previous ceasefires, the deal reached in Jeddah was signed by the parties and will be supported by a US-Saudi and internationally backed monitoring mechanism".
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In the hours before the ceasefire went into effect, the army conducted heavy air strikes across the capital Khartoum.
Artillery, rocket launchers and heavy machineguns were reportedly being used in the fighting in Khartoum and two cities across the Nile, Omdurman and Bahri.
There were also strikes by army aircraft on RSF positions.
The outbreak of fighting on Monday began during the morning hours, residents said. It died down before resuming in the afternoon and again shortly before sunset.
Army chief Gen Abdel Fattah Al Burhan and Gen Mohamed Dagalo of the RSF appear determined to win the conflict outright, something that is mainly viewed by analysts as unlikely given the nature of the urban warfare, now entering its sixth week.
In New York, the UN envoy to Sudan, Volker Perthes, warned of the increasing “ethnicisation” of the conflict and the possible effects on neighbouring nations.
“The growing ethnicisation of the conflict risks engulfing the country in a prolonged conflict, with implications for the region,” Mr Perthes told the UN Security Council on Monday.
“In West Darfur, clashes between the Sudanese Armed Forces and the Rapid Support Forces spiralled into ethnic violence on April 24. Tribal militias joined the fight and civilians took up arms to defend themselves.”
Such incidents have grown in recent weeks, he said.
“In parts of the country, fighting between the two armies or the two armed formations has sharpened into communal tensions, or triggered conflict between communities,” Mr Perthes said.
“Warning signs of tribal mobilisation are also reported in other parts of the country, particularly in South Kordofan.”
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The RSF and Gen Dagalo have their roots in the western region of Darfur, while the army's top brass traditionally come from the prosperous north of Sudan.
Darfur has been the scene of fierce fighting between the army and the RSF since last month.
The region was torn by civil war in the 2000s, when the RSF's forerunner, the Janjaweed militia, fought on the government's side against ethnic African rebels.
The Janjaweed, mostly drawn from local Arab tribes, regard themselves as victims of the perceived discrimination by Khartoum's political and military elite.
Fighting has killed at least 1,000 people and displaced more than one million, internally and into neighbouring countries such as Egypt, Chad and South Sudan.
Millions are trapped in Khartoum with little or no access to water, electricity or medicine. Most of the city’s healthcare centres have closed.
Much of the sprawling and dusty Nile-side city is deserted. Many residents have left while others are taking shelter in their homes.
Looting is widespread. Homes, banks, relief aid warehouses, stores and factories have all been hit by criminal gangs or possibly RSF fighters who have lost their bases or are left without supplies.
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Many of the RSF fighters have taken refuge in densely populated neighbourhoods, turning residents into human shields.
With most banks in Khartoum shut, warehouses and factories looted or burnt, and fuel in scarce supply, food has become increasingly difficult to obtain.
Aid agencies have increased their response to the crisis despite the challenges. Currently, 25 million people, more than half of Sudan’s population, are in need of assistance.
On Sunday, Martin Griffiths, UN undersecretary general for humanitarian affairs and the emergency relief co-ordinator, called for the “safe delivery of aid” and the restoration of essential services.
The warring parties signed an agreement in Jeddah on May 12 to protect civilians and let in aid shipments in.
Darfur has also suffered some of the worst violence. The UN said that hundreds of people have been killed there.
On Sunday, it said that all 86 gathering sites for displaced people in west Darfur’s city of Geneina “have reportedly been burnt to the ground”.
Darfur is still reeling from the conflict that began there in the early 2000s. At least 300,000 people died and 2.5 million were displaced during the war.
A 2021 coup staged by Gen Al Burhan and Gen Dagalo has created a security vacuum that sparked ethnic and tribal fighting in Darfur, claiming hundreds of lives and forcing tens of thousands to flee their homes.
The coup also upended Sudan's democratic transition and plunged the country into its worst economic crisis in living memory.