United Nations (UN) aid chief Martin Griffiths said on Wednesday he was seeking assurances from Sudan's warring factions on the safe delivery of aid after six trucks of humanitarian supplies were looted and air strikes in Khartoum undermined a new ceasefire.

"We will still require agreements and arrangements to allow for movement of staff and supplies," Griffiths said from Port Sudan, where many people have fled as the army and Rapid Support Forces (RSF) have battled for three weeks.

"We will need to have agreement at the highest level and very publicly, and we will need to deliver those commitments into local arrangements that can be depended on," he told reporters via videolink from the Red Sea port.

Air strikes were heard in the capital Khartoum on Wednesday even as warring factions agreed to a new seven-day ceasefire from Thursday, weakening chances for a lasting truce.

The conflict has created a humanitarian crisis, with about 100 000 people forced to flee with little food or water to neighbouring countries, the United Nations said.

Aid deliveries have been held up in a nation of 46-million people where about one-third had already relied on humanitarian assistance. A broader disaster could be in the making as Sudan's impoverished neighbours grapple with the influx of refugees.

Griffiths said he had been told by the World Food Programme that six of their trucks travelling to the western region of Darfur were looted en route despite assurances of safety and security. There was no immediate comment from WFP.

"It's a volatile environment, so we need those commitments," Griffiths said. "It's not as if we're asking for the moon. We're asking for the movement of humanitarian supplies, of people. We do this in every other country, even without ceasefires. It's a traditional humanitarian enterprise to go where others don't."

Saudi Arabia condemned what it said was the storming of the its cultural attache building in Khartoum. No further details were given.

Previous ceasefire agreements between the army and the paramilitary RSF, whose power struggle erupted into full blown conflict in mid-April, have ranged from 24 to 72 hours, but none have been fully observed.

Tens of thousands of people have left Khartoum and its adjoining cities at the confluence of the White and Blue Nile rivers, fearful of both air strikes and soldiers from the RSF.

Caught between army air strikes overhead and RSF soldiers on the ground, many citizens feel forced to take sides.

"If I hear the (army) air strikes I feel safe because at least I know the RSF won't come into my house," said Omdurman resident Salma, adding that the relentless fighting keeps her up at night. "I protested against (fallen autocrat Omar al-) Bashir and against army rule, but for now they're protecting me."

Several Khartoum neighbourhoods face severe water shortages due to blackouts, lack of fuel and damage to water supplies.

THIRD WEEK OF FIGHTING

South Sudan's foreign ministry said on Tuesday mediation championed by its president Salva Kiir had led both sides to agree a weeklong truce from Thursday to May 11 and to name envoys for peace talks. The current ceasefire was due to expire on Wednesday.

It was unclear, however, how the army chief, General Abdel Fattah al-Burhan, and RSF leader General Mohamed Hamdan Dagalo, better known as Hemedti, would proceed.

Army jets have been bombing RSF units dug into residential districts of the capital region.

Conflict has spread to Darfur where the RSF emerged from tribal militias that fought beside government forces to crush rebels in a civil war dating back 20 years.

The commanders of the army and RSF, who had shared power as part of an internationally backed transition towards free elections and civilian government, have shown no sign of backing down, yet neither side seems able to secure a quick victory.

Fighting now in its third week has engulfed Khartoum - one of Africa's largest cities - and killed hundreds of people. Sudan's Health Ministry reported on Tuesday that 550 people have died and 4 926 have been wounded.

Foreign governments were winding down evacuation operations that sent thousands of their citizens home. Britain said its last flight would depart Port Sudan on Wednesday and urged any remaining Britons wanting to leave to make their way there.