The sound of heavy artillery shelling and explosions was heard on Wednesday in Khartoum and the adjoining city of Omdurman, residents said.
The renewed fighting shattered a seven-day truce agreed to by the army and its rival, the paramilitary Rapid Support Forces.
Residents also told The National that heavy clashes were taking place between the two sides around the army-held base of Wad Sayedna, 20km west of Khartoum.
The base has an air strip believed to be used by jet fighters that have been pounding RSF positions since the fighting began on April 15.
Khartoum airport has been closed since the fighting began last month, with RSF fighters in near total control. That leaves Wad Sayedna the only airstrip in the greater Khartoum area available to the army.
Also on Wednesday, residents said the east Khartoum district of Manshiyah was being shelled and that jet fighters could be heard flying over the city.
The violence came after a truce mediated by the US and Saudi Arabia came into effect on Monday night. Fighting picked up in the hours before the truce began, but Khartoum was relatively quiet until fighting erupted again on Wednesday.
The ceasefire agreement, which provides for the delivery of humanitarian aid, is being monitored by Saudi Arabia and the United States as well as the warring parties. The deal came after five weeks of intense fighting in Khartoum and outbursts of violence in other areas of the country, including the western region of Darfur.
Aid workers say supplies and personnel arriving at Port Sudan on the Red Sea coast have been awaiting security permits and guarantees before they can leave.
Late on Tuesday, Saudi Arabia and the United States said members of the ceasefire monitoring mechanism, which includes representatives of the army and the RSF, had spoken to their chains of command about reported breaches of the truce.
The conflict has brought sustained air strikes and ground fighting to the capital region for the first time. Many residents are struggling to survive as they face prolonged water and power cuts, a collapse of health services and widespread lawlessness and looting.
The United Nations human rights chief called the situation in Sudan "heartbreaking" and said there were "very deeply troubling" accounts of sexual violence in Khartoum and Darfur with at least 25 cases reported so far and the real number likely to be much higher.
Sudan was facing severe humanitarian pressures even before the conflict broke out last month and forced more than 1.1 million people to flee their homes, threatening to destabilise the wider region.
More than 300,000 people have now fled Sudan to neighbouring countries, some of which are similarly impoverished and have a history of internal conflict. Many have crossed into Chad and Egypt in the past few days, Filippo Grandi, head of the UN refugee agency, UNHCR, said on Wednesday.
"Donor contributions to the refugee response plan remain scarce. We need more resources, urgently, to support countries hosting refugees," he said on Twitter.
The UN says the number of people requiring aid in Sudan has jumped to 25 million, more than half the population.
Additional reporting by Reuters
