Witnesses told AFP of "bombs falling and civilians being injured" in southern Khartoum, while others in the city's north reported "artillery fire", days after a US- and Saudi-brokered ceasefire collapsed.
Warplanes of the army led by General Abdel Fattah al-Burhan targeted positions of the paramilitary Rapid Support Forces (RSF) of General Mohamed Hamdan Dagalo, who responded with anti-aircraft fire, residents reported.
Both Burhan and his deputy-turned-rival Dagalo have pledged repeatedly to protect civilians and secure humanitarian corridors.
But civilians reported escalated fighting after the army quit ceasefire talks on Wednesday, including one army bombardment that a committee of human rights lawyers said killed 18 civilians in a Khartoum market.
Both sides have accused the other of violating the cease-fire and attacking civilians and infrastructure.
Washington slapped sanctions on the warring parties Thursday, holding both responsible for provoking "appalling" bloodshed.
In negotiations in Saudi Arabia last month, both parties had agreed to "enable responsible humanitarian actors, such as the Sudanese Red Crescent and/or the International Committee of the Red Cross to collect, register and bury the deceased".
But volunteers have found it difficult to move through the streets to retrieve the dead "due to security constraints," the Red Crescent said.
Aid corridors that had been promised as part of the truce never materialized, and relief agencies say they have managed to deliver only a fraction of needs, while civilians remain trapped.
The mission of the security forces is "to protect – not endanger – their fellow citizens," a US embassy statement said Saturday.
More than 700,000 people have fled Khartoum to other parts of Sudan that have been spared the fighting, in convoys of buses that regularly make their way out of the city.
But on their return, bus drivers were shocked to find they "were not allowed into the capital," one told AFP on Saturday, with others confirming authorities had blocked access since Friday, ordering the drivers to turn around.
On Friday, the army announced it had brought in reinforcements from other parts of the country to participate in "operations in the Khartoum area."
That sparked fears it was planning "a massive offensive", Sudan analyst Kholood Khair said.
So far, neither side has gained a decisive advantage. The regular army has air power and heavy weaponry, but analysts say the Rapid Support Forces (RSF) paramilitaries are more mobile and better suited to urban warfare.
The RSF announced Saturday their political adviser Youssef Ezzat had met Kenyan President William Ruto in Nairobi, as part of his visits to several "friendly countries to explain the developing situation in Sudan".
"We are ready to engage all the parties and offer any support towards a lasting solution," Ruto said on Twitter.
More than 1,800 people have been killed in the fighting, according to the Armed Conflict Location and Event Data Project.
Entire districts of Khartoum no longer have running water, electricity is only available for a few hours a week, and three quarters of hospitals in combat zones are not functioning.
The situation is particularly dire in the western region of Darfur, which is home to around a quarter of Sudan's population and never recovered from a devastating two-decade war that left hundreds of thousands dead and more than two million displaced.
The RSF is descended from the Janjaweed, a militia armed in 2003 to quash ethnic minority rebels in Darfur.
Amid what activists have called a total communication "blackout" in huge swathes of the region, hundreds of civilians have been killed, villages and markets torched and aid facilities looted, prompting tens of thousands to seek refuge in neighboring Chad.
According to aid group Doctors Without Borders (MSF), those crossing the border report horrific scenes of "armed men shooting at people trying to flee, villages being looted and the wounded dying" without medical care.
The UN says 1.2 million people have been displaced within Sudan and more than 425,000 have fled abroad – more than 100,000 west to Chad and 170,000 north to Egypt.