A 24-hour ceasefire took effect Saturday between Sudan's warring generals but, with fears running high it will collapse like its predecessors, US and Saudi mediators warned they may break off mediation efforts.

Three hours into the truce, residents in various areas of Khartoum said the shelling and air strikes that have rocked the capital daily since mid-April had halted, at least temporarily.

Civilians trapped in the battlegrounds in greater Khartoum and the flashpoint western region of Darfur are desperate for relief from the bloodshed but deeply sceptical about the sincerity of the generals.

An AFP photographer saw Khartoum residents taking advantage of the lull in fighting to stock up on fruit and other basic goods that have been in short supply.

Multiple truces have been agreed and broken since fighting started, and Washington had slapped sanctions on both rival generals after the last attempt collapsed at the end of May.

The nationwide truce announced by US and Saudi mediators on Friday took effect at 6:00 am (0400 GMT).

"Should the parties fail to observe the 24-hour ceasefire, facilitators will be compelled to consider adjourning" talks in the Saudi city of Jeddah which have been suspended since late last month, the mediators said.

Civilians voiced disappointment that the promised ceasefire was so limited in scope.

"A one-day truce is much less than we aspire for," said Khartoum North resident Mahmud Bashir. "We look forward to an end to this damned war."

Issam Mohamed Omar said he wanted an agreement that required fighters of the paramilitary Rapid Support Forces (RSF) who had occupied his home in Khartoum to leave so that he can return there from his temporary lodgings across the Nile in Omdurman.

"For me, a truce that doesn't kick the RSF out of the home they kicked (me) out of three weeks ago doesn't mean anything," he said.

- 'Incentives not changed' -

Sudan specialist Aly Verjee said he saw little reason why this truce should be honoured any more than its predecessors.

"Unfortunately, the incentives have not changed for either party, so it's hard to see that a truce with the same underlying assumptions, especially one of such short duration, will see a substantially different result, said Verjee, a researcher at Sweden's University of Gothenburg.

Upwards of 1,800 people have been killed in the fighting, according to the Armed Conflict Location and Event Data Project.

Nearly two million people have been displaced, including 476,000 who have sought refuge in neighbouring countries, the United Nations says.

The Saudi and US mediators said they "share the frustration of the Sudanese people about the uneven implementation of previous ceasefires".

The army, led by Abdel Fattah al-Burhan, said it has "agreed to the proposal", adding in a statement it "declares its commitment to the ceasefire".

The paramilitary RSF, commanded by Burhan's former deputy Mohamed Hamdan Daglo, said: "We affirm our full commitment to the ceasefire."

Both statements said the truce could support humanitarian efforts, while cautioning against violations by their opponents.

"If observed, the 24-hour ceasefire will provide an important opportunity... for the parties to undertake confidence-building measures which could permit resumption of the Jeddah talks," the US-Saudi statement said.

- UN backs envoy -

Friday's announcement came a day after Sudanese authorities loyal to Burhan declared UN envoy Volker Perthes "persona non grata", accusing him of taking sides.

UN chief Antonio Guterres later expressed support for Perthes, who is currently in the Ethiopian capital Addis Ababa for a series of talks.

Speaking through his spokesman, Guterres said "the doctrine of persona non grata is not applicable to or in respect of United Nations personnel" and is contrary to Khartoum's obligations under the UN charter.

The fighting has sidelined Perthes's efforts to revive Sudan's transition to civilian rule, which was derailed by a 2021 coup by the two generals before they fell out.

It has also complicated the coordination of international efforts to deliver emergency relief to the 25 million civilians that the United Nations estimates are in need.