The UN chief on Wednesday said "we failed" to stop war from erupting in Sudan, where persistent fighting between rival generals undermined efforts to firm up a truce, reported AFP.

"The UN was taken by surprise" by the conflict, because the world body and others were hopeful that negotiations would be successful, Antonio Guterres told reporters in Nairobi.

"To the extent that we and many others were not expecting this to happen, we can say we failed to avoid it to happen," the secretary general said.

"A country like Sudan, that has suffered so much... cannot afford a struggle for power between two people."

His remarks came as top UN humanitarian official Martin Griffiths was in Sudan one day after neighboring South Sudan announced that the warring sides had agreed "in principle" to a seven-day ceasefire.

Deadly violence broke out on April 15 between Sudan's de facto leader Abdel Fattah al-Burhan, who commands the regular army, and his deputy turned rival Mohamed Hamdan Daglo, who heads the paramilitary Rapid Support Forces (RSF).

At least 550 people have been killed and 4,926 wounded, according to the latest health ministry figures, which are likely to be incomplete.

More than 100,000 refugees have fled to neighboring countries in an exodus that has sparked warnings of a humanitarian "catastrophe" with implications for the entire region.

On Wednesday, Griffiths arrived in the Red Sea city of Port Sudan -- so far untouched by the fighting -- on an urgent mission to find ways to bring relief to the millions of Sudanese unable to escape.

Griffiths called for security guarantees "at the highest level" to ensure desperately needed aid deliveries to war-ravaged parts of the country.

"We know these general assurances need to be translated into specific commitments," he added.

Griffiths said he had been informed by the UN's World Food Programme that six trucks bringing aid to the country's western Darfur region had been "looted en route" Wednesday, "despite assurances of safety and security."

On Tuesday the foreign ministry of neighboring South Sudan announced that Burhan and Daglo "have agreed in principle for a seven-day truce from May 4th to 11th."

The two sides have yet to formally confirm the new ceasefire.

The warring sides have announced multiple truces but none has effectively taken hold. The current truce was extended on Sunday by a further 72 hours and is due to expire on Wednesday at 2200 GMT.