"We were woken by explosions and heavy artillery fire," one resident of Khartoum's sister city of Omdurman told AFP.
During the night, two huge explosions were heard across greater Khartoum, residents of multiple districts said.
Witnesses said the army unleashed intense air bombardment in the center of Khartoum and around the presidential palace. The rival Rapid Support Forces (RSF) paramilitary said the palace, which it claims to control, was hit by an air strike and destroyed, but an army source denied the claim.
More than 750 people have been killed since the fighting erupted on April 15, the majority of them civilians.
Nearly 150,000 refugees have crossed into neighboring countries, while 700,000 people have been displaced inside Sudan.
Lawlessness has taken hold in Khartoum and the two adjoining cities of Omdurman and Bahri, witnesses said. "The biggest danger is the spread of robbery and looting and the total absence of the police and the law," said Ahmed Saleh, 45, from Bahri.
Homes, shops and warehouses have all been targeted, residents said. Sudan's Banks Union condemned burglary and vandalism at some branches, saying banks were seeking to restore services if conditions allowed.
The fighting in Khartoum, which erupted April 15, has prompted hundreds of thousands of people to flee their homes and triggered an aid crisis. The number of people internally displaced within Sudan more than doubled in a week to 700,000, the U.N.'s migration agency said.
The two forces, which have failed to abide by repeated truce deals, sent representatives to talks in the Saudi port city of Jeddah on Saturday.
De-facto Sudan leader General Fattah al-Burhan said in a phone interview with an Egyptian TV station on Monday that the talks in Jeddah were aimed at relieving pressure on civilians, not at any political settlement. He accused the RSF of cutting off power and telecoms in areas they controlled.
The RSF has said in statements that it is seeking to keep services running in Khartoum and has rejected reports of looting and other abuses. It said on Tuesday that the army had been hitting residential areas with air strikes.
In the first report on the talks thus far, the Saudi foreign ministry said on Tuesday that the negotiations aimed to reach "an effective short-term ceasefire," Saudi state TV Al-Ekhbariya said.
U.N. aid chief Martin Griffiths proposed the warring parties back a declaration guaranteeing safe passage of aid supplies and the proposal has been discussed in Jeddah, a U.N. spokesperson said.
Griffiths "hopes the declaration can be endorsed as soon as possible so that the relief operation can scale up swiftly and safely to meet the needs of millions of people in Sudan," Deputy U.N. spokesperson Farhan Haq told reporters.
The United Nations estimates that 5 million additional people will need emergency assistance inside Sudan while 860,000 are expected to flee to neighboring states that were already in crisis at a time when rich countries have cut back on aid.
U.S. Secretary of State Antony Blinken said on Tuesday the U.S. will not give up the goal of putting Sudan back on track to civilian democratic governance, adding that Washington is working in Jeddah for a ceasefire and agreement on humanitarian aid.
