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.
Since fighting between Sudan's warring generals erupted on April 15, volunteers have buried 102 unidentified bodies in the capital's Al-Shegilab cemetery and 78 more in cemeteries in Darfur, the Sudanese Red Crescent said in a statement.
Both regular army chief Abdel Fattah al-Burhan and his deputy-turned-rival, paramilitary commander Mohamed Hamdan Daglo, have issued repeated pledges to protect civilians and secure humanitarian corridors.
But civilians reported escalated fighting after the army quit ceasefire talks on Wednesday, including a single army bombardment that killed 18 civilians in a Khartoum market, according to a committee of human rights lawyers.
Both sides have accused the other of violating the ceasefire, as well as attacking civilians and infrastructure.
Washington slapped sanctions on the warring parties Thursday, holding them both responsible for provoking "appalling" bloodshed.
Not allowed in
In negotiations in Saudi Arabia last month, the warring 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 pick up the dead, "due to security constraints", the Red Crescent said.
Aid corridors that had been promised as part of the truce never materialised, and relief agencies say they have managed to deliver only a fraction of needs, while civilians remain trapped.
Over 700,000 people have fled the capital to other parts of Sudan that have been spared the fighting, in convoys of buses that regularly make their way out of Khartoum.
But on their way back, 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 back.
The army had earlier on Friday announced it had brought in reinforcements from other parts of Sudan to participate in "operations in the Khartoum area".
That sparked fears it was planning "to launch a massive offensive," according to Sudan analyst Kholood Khair.
So far neither side has gained a decisive advantage. The regular army has air power and heavy weaponry, but analysts say the paramilitaries are more mobile and better suited to urban warfare.
The RSF announced Saturday its political advisor Youssef Ezzat had met with 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.
Blackout in Darfur
More than 1,800 people have been killed in the fighting, according to the Armed Conflict Location and Event Data Project.
Entire districts of the capital 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.
Renewed clashes were reported on Saturday in the town of Kutum in North Darfur, according to witnesses.
Amid what activists have called a total communications "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 neighbouring 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 access to 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.