Observers say that while sporadic artillery fire still echoed across the capital Wednesday, "fighting in Khartoum appeared to be less intense" since the truce entered into force late Monday.
Washington and Riyadh voiced "concern" however that the warring sides had sought to gain military advantage in the lead-up to the truce and pointed to reports "indicating that both sides violated the agreement."
U.S. Secretary of State Antony Blinken has warned that "if the cease-fire is violated, we'll know" and pledged to "hold violators accountable through our sanctions and other tools at our disposal."
The two nations stressed that preparations were underway "to deliver lifesaving assistance" to the people of Sudan, who have endured more than five weeks of fighting that has claimed more than 1,000 lives.
The chaos has left millions hunkering down in their homes to hide from the combatants and roaming looters amid intense summer heat, power blackouts and desperate shortages of food, medicines and other staples.
A mass exodus of Sudanese has meanwhile continued into neighboring countries, including Chad, Egypt and South Sudan, sparking regional fears the conflict will spread across borders because of transnational ethnic ties.
Fighting has also displaced more than one million people inside the country since it erupted on April 15, the International Organization for Migration said Wednesday.
A further 319,000 people have sought refuge in neighboring countries, the U.N. refugee agency UNHCR added. It said more than 132,000 refugees had arrived in Egypt, 80,00.0 in Chad and 69,000 in South Sudan.
Fighting has been especially deadly in the western Darfur region where Sudan's then dictator Omar al-Bashir unleashed the notorious Janjaweed militia in the early 2000s and some 300,000 people were killed.
The latest violence in El-Geneina, West Darfur state, has left all 86 reception centres for displaced people "completely burned down" and 85,000 people there on the move yet again, U.N. agencies have reported.
"The whole country has been taken hostage", said the U.N.'s expert on human rights in Sudan, Radhouane Nouicer. "People feel alone and abandoned."
Sudan expert Alex de Waal warned that the "trajectory of state collapse" is now threatening "to turn Sudan as a whole, including Khartoum, into something that resembles the Darfur of 10-15 years ago".
He pointed to warring Rapid Support Forces leader Mohamed Hamdan Dagalo's roots in the Janjaweed and said that this "is the environment within which "Hemeti" [Dagalo's nickname] thrived, where money and guns determine everything - this is the future of Sudan if this carries on."
Dagalo's forces are in pitched conflict with the Sudan army, led by Gen. Abdel Fattah al-Burhan.
Medical aid workers meanwhile voiced alarm about dire shortages as fighting has left most hospitals destroyed, ransacked and even used as fire bases, particularly in Khartoum and Darfur.
"After the looting of one of our medical warehouses in Khartoum, fridges were unplugged and medicines removed," said Jean-Nicolas Armstrong Dangelser of aid group Doctors Without Borders. "The entire cold chain was ruined so the medicines are spoiled and can't be used to treat anyone."
He added: "We are experiencing a violation of humanitarian principles and the space for humanitarians to work is shrinking on a scale I've rarely seen before."
Like many Sudanese citizens, Yasser Abdelaziz, a civil servant in the northern town of Shendi, said he fears a war worse than other Middle East conflicts and more like the turmoil seen elsewhere in the Horn of Africa.
"I'm afraid that the scenario to come will not be Syria, Libya or Yemen," he said, "but the Somali scenario, with people driven by racism and tribalism."