The United Nations said on Wednesday it 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," U.N. aid chief Martin Griffiths said from Port Sudan, where many people have fled as the army and Rapid Support Forces (RSF) have battled for more than two 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 neighboring 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 neighbors grapple with the influx of refugees. Griffiths said he had been told by the World Food Program 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." In Nairobi, UN Secretary-General Antonio Guterres told a press conference that the entire international community must clearly tell the army chief, General Abdel Fattah al-Burhan, and paramilitary leader General Mohamed Hamdan Dagalo, better known as Hemedti, that the situation is unacceptable. He also said the two generals must face pressure to agree on a ceasefire, and establish a political dialogue and a transition to a civilian government. Previous ceasefire agreements between the army and the 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 President Omar al-) Bashir and against army rule, but for now they're protecting me." Several Khartoum neighborhoods 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 Burhan and Hemedti would proceed. Army jets have been bombing RSF units dug into residential districts of the capital region. 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 has engulfed Khartoum - one of Africa's largest cities - and killed hundreds of people. Sudan's Health Ministry said on Tuesday 550 people have died, with 4,926 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.