In both the north and south of the capital, Khartoum, key bases of commander Mohamed Hamdan Dagalo's Rapid Support Forces came under attack by troops loyal to army chief Abdel Fattah al-Burhan, residents told AFP.
A witness said there was "heavy artillery fire from army camps" in northern Khartoum, on the 47th day of a war that researchers said has claimed at least 1,800 lives.
Another witness reported "artillery blasts on the RSF camp in al-Salha" in southern Khartoum -- the largest paramilitary base and arsenal stock in the capital
The army walked out of peace talks "because the rebels have never implemented a single one of the provisions of a short-term ceasefire which required their withdrawal from hospitals and residential buildings", a Sudanese government official said on condition of anonymity because he was not authorized to speak to the press.
The army said the paramilitary Rapid Support Forces (RSF) had also "repeatedly violated the truce," the official added.
U.S. and Saudi mediators said late Monday that the warring parties had agreed to extend by five days a humanitarian truce they had frequently violated over the previous week.
The mediators admitted the truce had been "imperfectly observed" but said the extension "will permit further humanitarian efforts."
But despite the pledges of both sides, fighting flared again on Tuesday both in greater Khartoum and in the flashpoint western region of Darfur.
"The army is ready to fight until victory," army chief Abdel Fattah al-Burhan declared during a visit to troops in the capital.
The RSF, led by al-Burhan's deputy-turned-foe Mohamed Hamdan Dagalo, said they will "exercise their right to defend themselves" and accused the army of violating the truce.
Sudan analyst Aly Verjee said the mediators were eager to avoid a complete breakdown of the talks, for fear of a major escalation on the ground.
"The mediators know that the situation is bad but they do not want to state that a ceasefire is gone for fear that the situation would then become even worse," said Verjee, a researcher at Sweden's University of Gothenburg.
"The hope is that by keeping the parties talking, the prospects of arrangements that are better respected will eventually improve."
Since fighting erupted between the rival security forces on April 15, more than 1,800 people have been killed, according to the Armed Conflict Location and Event Data Project.
The United Nations says more than a million people have been internally displaced and nearly 350,000 have fled abroad, including over 170,000 to Egypt.
More than half the population - 25 million people - are now in need of aid and protection, the U.N. says.
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 out of service.
Many families have continued to hide out in their homes, rationing water and electricity while trying desperately to avoid stray gunfire in the city of more than five million people - nearly 700,000 of whom have fled, according to the United Nations.
In Darfur, on Sudan's western border with Chad, continued fighting "blatantly disregards ceasefire commitments," said Toby Harward, of the U.N. refugee agency UNHCR.
The persistent fighting has impeded delivery of the aid and protection needed by a record 25 million people, more than half the population, according to the U.N..
The U.N. has warned for weeks that fighting in Darfur's major cities has also drawn in former rebel and militia fighters recruited along ethnic lines during the region's devastating conflict in the mid-2000s.
Darfur's pro-army governor Mini Minawi, a former rebel leader, has urged citizens to "take up arms" to defend their property.
Sudan could descend into "total civil war", warned the Forces for Freedom and Change, the main civilian bloc ousted from power by al-Burhan and Dagalo in a 2021 coup before the two men fell out.