The Sudanese Armed Forces and the paramilitary Rapid Support Forces (RSF) agreed to the ceasefire "following intense negotiations," U.S. Secretary of State Antony Blinken said in a statement shortly before the truce took effect at 2200 GMT Monday.
The fighting has pitted forces loyal to army chief Abdel Fattah al-Burhan against those of his former deputy Mohamed Hamdan Dagalo, who commands the RSF.
Blinken said the U.S. would coordinate with regional, international and Sudanese civilian interests to create a committee that would oversee work on a permanent ceasefire and humanitarian arrangements.
Previous bids to pause the conflict failed to take hold, but both sides confirmed they had agreed to the three-day halt.
"This ceasefire aims to establish humanitarian corridors, allowing citizens and residents to access essential resources, healthcare and safe zones, while also evacuating diplomatic missions," the RSF tweeted.
Experts have long drawn links between the RSF and Russian mercenary group Wagner. Secretary Blinken earlier on Monday voiced "deep concern" that Wagner risked aggravating the war in Sudan.
In a statement on Facebook, the army said it would abide by the ceasefire on condition its rivals did so.
The Forces of Freedom and Change - the main civilian bloc which the two generals ousted from power in a 2021 coup - said the truce would allow for "dialogue on the modalities of a permanent ceasefire."
Yassir Arman, a leading figure in the FFC, urged humanitarian groups and the international community to help restore water and electricity, and send generators to hospitals.
"There are bodies scattered in streets and sick people who cannot find medicine, no water nor electricity. People should be allowed to bury their dead during the ceasefire," he said.
The U.N. humanitarian office (OCHA) said shortages of food, water, medicines and fuel were becoming extremely acute, with prices for basic goods including bottled water rocketing.
Ten days of heavy fighting, including air strikes and artillery barrages, have killed hundreds of people, many of them civilians, and left some neighborhoods of greater Khartoum in ruins.
The capital, a city of five million, has endured "more than a week of unspeakable destruction," Norway's ambassador Endre Stiansen wrote on Twitter after his evacuation.
At least 459 people have been killed and more than 4,000 wounded in the fierce fighting across Africa's third-biggest country, according to UN agencies.
More than 4,000 people have fled Sudan in foreign-organized evacuations that began on Saturday, including by sea to Saudi Arabia and by aircraft to Jordan and Cyprus.
"The quick evacuation of Westerners means that the country is on the brink of collapse. But we expect a greater role from them in supporting stability by pressuring the two sides to stop the war," said Suleiman Awad, a 43-year-old academic in Omdurman.
One 65-vehicle convoy took dozens of children, along with hundreds of diplomats and aid workers, on an 800km, 35-hour journey in searing heat from Khartoum to Port Sudan on the Red Sea.
But millions of Sudanese are unable to flee the country, which has a history of military coups. For those remaining in Sudan, where a third of its 46 million people needed aid even before the violence, the situation was increasingly bleak.
There were acute shortages of food, clean water, medicines and fuel and limited communications and electricity, with prices skyrocketing, said deputy U.N. spokesperson Farhan Haq.
There is another risk: The World Health Organization warned Tuesday that fighters in conflict-ravaged Sudan had occupied a central public laboratory holding samples of diseases including polio and measles, creating an "extremely, extremely dangerous" situation.
"There is a huge biological risk associated with the occupation of the central public health lab... by one of the fighting parties," Nima Saeed Abid, the WHO's representative in Sudan, told reporters in Geneva via video-link.
He did not say which of the fighting parties had taken over the laboratory.
This report was sourced from Reuters and Agence France-Presse.