Hundreds of people have been killed and thousands wounded during 16 days of battles since disputes between the Sudanese army and the paramilitary Rapid Support Forces (RSF) erupted into conflict on April 15.
The crisis has unleashed a humanitarian disaster, damaged swathes of Khartoum, risked drawing in regional powers and reignited conflict in the Darfur region.
Many fear for their lives in the power struggle between the army chief and RSF head, who shared control of the government after a 2021 coup but fell out over a planned transition to civilian rule.
Both sides agreed on Sunday to extend a much-violated truce by 72 hours, and the UN told Reuters the rival forces might hold ceasefire talks in Saudi Arabia.
But air strikes and artillery rang out on Monday as smoke hung over Khartoum and neighbouring cities.
UN official Raouf Mazou said the body's refugee agency was planning for an exodus of 815,000 people including 580,000 Sudanese as well as foreign refugees now living in the country.
The country's population numbers 46 million.
Some 73,000 have already left Sudan, he said.
Egypt reported 40,000 Sudanese had crossed its border, and those who made the journey said conditions were arduous.
Others have gone to Chad, South Sudan and Ethiopia, or sailed across the Red Sea to Saudi Arabia on evacuation boats.
At least 528 people have been killed and 4599 wounded, the health ministry said.
The UN has reported a similar number of dead but believes the real toll is much higher.
Foreign governments pulled out their citizens during the past week in air, sea and land operations, although several countries ended efforts.
The US government said on Monday its convoys from Khartoum to the Red Sea harbour of Port Sudan evacuated more than 700 people at the weekend.
Britain said it was exploring ways to provide humanitarian assistance to Sudan along with its international allies, the UN and aid organisations after evacuating almost 2200 people.
Those Sudanese who stayed behind faced hardship and danger.
Power and water supplies are uncertain, there is little food or fuel, most hospitals and clinics are out of service and soaring transport costs are making it even harder to leave.
The UN and other aid organisations have cut services, although the World Food Programme said it was resuming operations in more secure areas on Monday after staff were killed early in the war.
"The scale and speed of what is unfolding in Sudan is unprecedented," said Martin Griffiths, a senior UN official for humanitarian and emergency relief who will visit Sudan on Tuesday.
Both sides said on Monday they were making progress without commenting directly on the ceasefire violations.
The army said it had cut RSF's combat effectiveness by half and stopped it trying to reinforce its positions in the capital.
The RSF said it still controls the main locations of Khartoum and was itself beating back army reinforcements.
Reuters could not verify either side's claims.
