Rockets hit a market in Sudan's capital, Khartoum, killing 18 people and leaving more than 100 wounded, doctors and residents say.
The fighting between rival military forces comes as truce talks mediated by the US and Saudi Arabia collapsed.
Wednesday's violence around a market in Mayo in the south of Khartoum included artillery fire and aerial bombardment.
It caused the most civilian casualties in a single incident in the capital since the war began in April.
This brings the civilian death toll over seven weeks to at least 883, according to official counts - though the real number is likely to be far higher.
Neighbourhood organisations - which have been helping Khartoum's residents get food and medicine - described it as a catastrophic situation and appealed for doctors and blood donations.
With so much of the violence taking place in urban areas civilians are in constant danger.
On Tuesday, the army and its rivals from the Rapid Support Forces (RSF) had agreed to extend last week's humanitarian ceasefire deal for another five days, in talks brokered by the US and Saudi Arabia.
But the next day the army withdrew from the talks, alleging the RSF was not committed to the terms.
The US says both sides have violated the ceasefire - adding it remained ready to help mediate a truce when they were serious about ending the violence.
The ceasefire had allowed some urgent aid to reach around two million people, but the continued insecurity had "prevented delivery to many more and blocked operations to restore essential services", a US State Department spokesperson said.
New sanctions have also been announced by the US Treasury Department aimed at cutting vital funding to the two warring sides.
The four companies listed include one that owns gold mines and is controlled by the head of the RSF. A multi-billion dollar arms manufacturer that supplies the army is also on the list.
Given that Sudan has faced years of US-imposed restrictions in the past, it is unclear if any of the companies have or need any links to America.
According to the UN, 25 million people, more than half the population of Sudan, are now in need of humanitarian aid and protection.
With talks no longer taking place there is a fear of the fighting escalating - heavy fire was reported on Thursday morning across the River Nile from Khartoum in the cities of Bahri and Omdurman.
"We are being terrorised by the sounds of heavy artillery around us. The house has been shaking," a 49-year-old resident of Omdurman told the Reuters news agency.
The UN World Food Programme (WFP) says its warehouses in El Obeid in North Kordofan state, with food for 4.4 million people, are coming under attack.
"It is unconscionable to steal from the hungry. This must stop," WFP chief Cindy McCain tweeted.
The fighting, which has also been fierce in Sudan's western Darfur region, is a direct result of a vicious power struggle between the two generals who led the 2021 coup - army chief Abdel Fattah al-Burhan and RSF commander Mohamed Hamdan Dagalo, better known as Hemedti.