Sporadic artillery fire still echoed through Sudan's capital Tuesday but residents said fighting had calmed following a US and Saudi-brokered ceasefire, raising faint hopes in the embattled city.

Witnesses in Khartoum reported a welcome respite after a rocky start to the one-week humanitarian truce which took effect the previous evening only to be quickly marred by more gunshots and blasts.

By around noon on Tuesday, witnesses reported a relative calm had taken hold, both in greater Khartoum and in the Darfur region's cities of Nyala and El Geneina, which have been among the other main battlegrounds.

"We have not heard shelling in our neighbourhood since last night," said a witness in southern Khartoum, who told AFP the last airstrike was five minutes before the truce formally started at 9:45 pm (1945 GMT) on Monday.

"If the ceasefire is violated, we'll know," US Secretary of State Antony Blinken said in a video message. "And we will hold violators accountable through our sanctions and other tools at our disposal."

More than five weeks of war have pitted the army, led by Sudan's de facto leader Abdel Fattah al-Burhan, against the paramilitary Rapid Support Forces of his former deputy Mohamed Hamdan Daglo.

The battles since April 15 have killed an estimated 1,000 people, forced more than a million to flee their homes and sparked mass evacuations of foreigners and major refugee flows into neighbouring counties.

People have run low on water, food and basic supplies, and more than half the population, 25 million people, are in need of humanitarian aid, according to the UN.

As the uneasy silence held in Khartoum, residents desperately hoped for a pause in combat to allow in life-saving humanitarian aid, and to enable more people to flee the strife-torn city of five million.

- 'Broken promises' -

A series of previous ceasefires were quickly shattered, and a foreign aid group Monday voiced frustration about the crisis which has piled new misery on the already poverty-stricken northeast African nation.

"Beyond official announcements, Sudan is still pounded and bombarded, with millions of civilian lives at risk," Karl Schembri of the Norwegian Refugee Council wrote on Twitter.

"We've had over a month of broken promises and empty words while humanitarian colleagues were killed, together with children and others and hospitals destroyed."

Volker Perthes, the United Nations envoy to Sudan, told the UN Security Council on Monday that "fighting and troop movements have continued even today, despite a commitment by both sides not to pursue military advantage before the ceasefire takes effect".

While no previous truce has held, the United States and Saudi Arabia said this agreement was different because it was "signed by the parties" and would be supported by a "ceasefire monitoring mechanism".

Neither side has yet blamed the other for violating the truce, as they did within minutes after the previous ceasefires unravelled.

- 'Victory or martyrdom' -

Hours before the truce was scheduled to start, Daglo released a voice message on social media in which he told his fighters: "It is either victory or martyrdom, and victory will be ours."

Addressing reported violations by his forces -- including rampant looting, targeting of civilians and attacks on churches -- he blamed "coup plotters" in the army.

Major fighting has rocked the western Darfur region near Chad, where the UN has reported hundreds of civilians killed in the West Darfur capital El Geneina.

Perthes in his Security Council address warned that "the conflict risks to expand and prolong ... with implications for the region."

"In parts of the country, fighting between the two armies or the two armed formations has sharpened into communal tensions, or triggered conflict between communities," he said, after reports of civilians being armed in Darfur.

Sudan has a long history of military coups and the army in 2019 overthrew the veteran Islamist autocrat Omar al-Bashir after mass protests against his rule.

Sudanese were promised a gradual transition toward civilian rule, but Burhan and Daglo staged another coup in October 2021 before simmering tensions between the two men flared into the current round of bloody fighting.