Some foreign nationals began evacuating from Sudan Saturday after a week of strife that has killed hundreds of civilians, even as the army used air strikes against a rival paramilitary force during battles in central Khartoum.

The bloody onslaught of urban warfare has trapped large numbers in the Sudanese capital. The airport has been repeatedly targeted and many residents have been unable to leave their homes or get out of the city to safer areas.

The United Nations and foreign states have urged rival military leaders to honour declared ceasefires that have mostly been ignored, and to open safe passage both for fleeing civilians and for the supply of badly needed aid.

With the airport closed and skies unsafe, thousands of foreigners - including embassy staff, aid workers and students in Khartoum and elsewhere in Africa's third largest country - have also been unable to get out.

The army said early Saturday it would provide safe pathways to evacuate nationals from the US, Britain, France and China, while Saudi Arabia and Jordan were already evacuating via Port Sudan on the Red Sea. It said airports in Khartoum and Darfur's biggest city Nyala were problematic.

By late Saturday afternoon, Saudi Arabia said it had evacuated 157 Saudis and people of other nationalities, broadcasting footage of people on a naval ship, and Kuwait said some of its citizens had arrived in Jeddah. Jordan said it had started evacuating 300 citizens.

One foreign diplomat who asked not to be identified said some diplomatic staff in Khartoum were hoping for evacuation by air from Port Sudan in the next two days. The US embassy warned Americans that there was "incomplete information" about convoys heading out of Khartoum and travel would be at individuals' own risk.

The army, under Abdel Fatteh al-Burhan and the rival Rapid Support Forces (RSF), headed by Mohamed Hamdan Dagalo, known as Hemedti, have so far failed to observe ceasefires agreed almost daily since hostilities broke out on April 15.

Saturday's fighting breached what was meant to be a three day truce from Friday to allow citizens to reach safety and visit family during the holiday of Eid al-Fitr. Both sides accused the other of breaching the truce.

The RSF said it was ready to partially open all airports to allow evacuations. However, the status of Sudan's airports is unclear. Khartoum Airport said on Twitter that Sudan's airspace would stay closed to traffic.

International medical charity Medecins Sans Frontieres (MSF) appealed for safe passage to supply hospitals and allow medical staff to work freely.

"We need a space where we can supply different locations... we need ports of entry where we can bring specialist trauma staff and medical supplies," said MSF Sudan operations manager Abdalla Hussein.