Strikes from the air, tanks and artillery shook Sudan's capital of Khartoum on Friday and a heavy bombardment pounded the adjacent city of Bahri, witnesses said, even though the army and a rival paramilitary force agreed to extend a truce by 72 hours.

Hundreds have been killed and tens of thousands have fled for their lives in a power struggle between the army and the Rapid Support Forces (RSF) that erupted on April 15 and disabled an internationally backed transition to democracy.

In persisting ceasefire violations that the United States called worrying, heavy gunfire and detonations rattled residential neighbourhoods of the capital region where fighting has been concentrated over the past week.

Thick smoke was rising above two areas of Bahri.

"The situation this morning is very scary. We hear the sounds of planes and explosions. We don't know when this hell will end," said Bahri resident Mahasin al-Awad, 65. "We're in a constant state of fear for ourselves and our children."

Sudan's army has been directing air strikes with jets or drones on RSF forces spread out in neighbourhoods across the capital where many residents are pinned down by urban warfare with little access to food, fuel, water and electricity.

In a statement, the RSF accused the army of violating a US- and Saudi-brokered truce pact by carrying out air strikes on its bases in Omdurman, Khartoum's sister city at the confluence of the Blue and White Nile rivers, and Mount Awliya.

A Turkish evacuation plane came under fire as it was landing at Wadi Seyidna airport in Omdurman on Friday but there were no injuries, Turkey's defence ministry said.

Shortly before, Sudan's army accused the RSF of firing at the plane, damaging its fuel system which was being repaired after the aircraft managed to land safely. The RSF denied attacking the plane, accusing the army of "spreading lies".

The RSF said the army's air strikes were impeding evacuation efforts by foreign diplomatic missions.

Fighting has also spread to the Darfur region, where conflict has simmered since a brutal civil war erupted two decades ago, and threatens to compound instability across a volatile swathe of Africa between the Sahel and the Red Sea.

UN humanitarian office spokesperson Ravina Shamdasani said at least 96 people had been killed in Darfur since Monday by inter-communal violence rekindled by the army-RSF conflict.

"It is deeply alarming that inmates have been released from or escaped from a number of prisons, and we are very worried about the prospect of further violence amid a generalised climate of impunity," she told a briefing in Geneva.

The Sudan Doctors Union said on Friday that at least 387 civilians had been killed and 1 928 injured in the fighting since April 15, which has also caused a humanitarian crisis.

Relief agencies have been largely unable to distribute food to the needy in Africa's third-largest country, where a third of its 46-million people were already reliant on donations.

MORE EVACUATIONS, REFUGEES

Saudi Arabia said two more evacuation ships had arrived in Jeddah, across the Red Sea from Sudan, on Friday carrying 252 people in all from various countries, raising to more than 3 000 the total Riyadh has rescued, most of them other nationalities.

Among Sudan's neighbours, Egypt said it had taken in 16 000 people, 20 000 had entered Chad and 10 000 had crossed into South Sudan, which won independence from Khartoum in 2011 after decades of shattering civil war.

"The clock is ticking fast. We're working as swiftly as we can ... to deploy where it's possible and safe to do so," UN refugee agency (UNHCR) spokesperson Matthew Saltmarsh said.

"This region has suffered decades of hardship, deprivation and conflict. Given the sheer scale of the country with its seven borders and the speed at which events are moving..., precise (refugee) numbers are still hard to come by."

Dodging the fighting wherever possible, Sudanese civilians have been fleeing Khartoum, one of Africa's largest cities which had long been untouched by Sudan's string of civil wars.

The army and RSF said earlier they had agreed to extend the ceasefire to midnight on Sunday to prolong one that expired on Thursday night that had brought a partial lull and allowed diplomatic evacuations to gather pace.

The news was welcomed by the United Nations, African Union, African trade bloc IGAD and the so-called quad countries of the US, Britain, Saudi Arabia and the United Arab Emirates. In a statement, they emphasised that the goal was a more durable ceasefire and untrammelled access for humanitarian operations.

But the White House said on Thursday it was deeply concerned by the myriad ceasefire violations and that conditions could worsen at any moment.

Army chief General Abdel Fattah al-Burhan's office said on Friday he had received calls of support for restoring calm from several regional leaders including South Sudan's president and Ethiopia's prime minister, the Saudi foreign minister and a diplomatic grouping that includes the United States and Britain.

Friction had been building for months between Sudan's army and the RSF, whose 2021 coup came two years after a popular uprising toppled long-ruling Islamist autocrat Omar al-Bashir.