Photo Shows an aerial view of black smoke rising over Khartoum. AFP
More than 500 people have been killed since battles erupted on April 15.
Millions of Sudanese around the capital have since hidden in their homes with dwindling food, water, and electricity as warplanes on bombing raids have drawn heavy fire from anti-aircraft guns.
"Warplanes are flying over southern Khartoum and anti-aircraft guns are firing at it," said one resident, while another witness told AFP he was also hearing "loud gunfire" in the area.
The two rivals have agreed multiple, poorly observed ceasefires, and extended the latest formal truce on Sunday by 72 hours, with each side repeatedly blaming the other for the frequent violations.
Millions of Sudanese are trapped in the country, where aid workers are among the dead, humanitarian facilities have been looted, and foreign aid groups have been forced to essentially halt all aid operations.
Top UN humanitarian official Martin Griffiths said Sunday he was heading to the region to help "bring immediate relief to the millions of people whose lives have turned upside down overnight".
"The humanitarian situation is reaching breaking point," he said. "Goods essential for people's survival are becoming scarce in the hardest-hit urban centres, especially Khartoum."
"The cost of transportation out of worst-hit areas has risen exponentially, leaving the most vulnerable unable to locate to safer areas."
50,000 flee overland
Some 50,000 people have fled the raging conflict, seeking refuge in neighbouring countries including Chad, Egypt, and the Central African Republic, said the UN refugee agency.
The fighting has also triggered a mass exodus of foreigners and international staff, with countries the world over launching frantic evacuations by land, sea, and air.
Further complicating the battlefield, Central Reserve Police were being deployed across Khartoum to "protect citizens' properties" from looting, the Sudanese police said, confirming an army statement.
The RSF had warned police against joining the fight.
The US Treasury Department last year sanctioned the Central Reserve for "serious human rights abuses" related to its use of "excessive force" against pro-democracy protests in October 2021.
UN facilities looted
At least 528 people have been killed and almost 4,600 people wounded in the violence, according to Sudan's health ministry, but the death toll is feared to be far higher.
Fighting has also spread across Sudan, especially in the long-troubled Darfur region, where witnesses reported intense conflict and looting.
At least 96 people were reported killed in El Geneina, West Darfur, the UN said.
The UN World Food Programme has warned the unrest could plunge millions more into hunger in a country where 15 million people already needed aid to stave off famine.
Only 16 percent of health facilities are functioning in Khartoum, according to the World Health Organisation, with many facilities shelled.
On Sunday, a first Red Cross plane brought eight tonnes of humanitarian aid from Jordan to Port Sudan, which is so far untouched by the fighting and has served as an evacuation hub.
The aid included surgical material and medical kits to stabilise 1,500 patients.