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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.
"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."
The UN also called on Wednesday for security guarantees at "the highest level" to ensure desperately needed aid deliveries in conflict-hit Sudan, after six trucks carrying food aid to the Darfur region were looted.
Top UN humanitarian official Martin Griffiths insisted on the need "to be sure that we have the commitments publicly, clearly given by militaries, to protect humanitarian systems to deliver".
"We will need to have agreement at the highest level and very publicly," he told journalists via video link from Sudan.
Sudan's warring military factions agreed to a new and longer seven-day ceasefire from Thursday, neighbour and mediator South Sudan said, even as more air strikes and shooting in the Khartoum region undercut their latest supposed truce.
Both sides agreed to a week-long truce from Thursday to May 11 and to name envoys for peace talks. The current ceasefire was due to expire on Wednesday.
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.
Agencies contributed to this story.