United Nations Secretary-General Antonio Guterres on Wednesday admitted that "we failed" to stop the 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 toward a civilian transition would be successful, 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 UN chief indicated.

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

Deadly urban clashes broke out on April 15 between Abdel Fattah Al-Burhan, who commands the regular army, and his deputy Mohammad Hamdan Dagalo, 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 incomplete.

Sudan expert Ernst Jan Hogendoorn, writing for the Atlantic Council, considered that international and regional leaders must "begin to strategically apply pressure" by freezing bank accounts and blocking business activities of Sudanese leaders and their forces.

'Heartbreaking', 'catastrophic' situation

Guterres spoke on the same day his top humanitarian official, Martin Griffiths, was in Sudan after neighboring South Sudan announced on Tuesday that the warring sides had agreed "in principle" to a seven-day ceasefire from May 4.

In a statement early Thursday, the Sudanese army said it had "accepted" this proposed ceasefire extension while calling for "an African solution to the problems of the continent."

But by midnight Wednesday, the RSF had not commented on the ceasefire, and the army stressed in its statement that all its commitments were conditional on "respect for the truce" by the other side.

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.

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

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

Both Griffiths and UN's Sudan envoy Volker Perthes spoke to Burhan and Dagalo over the phone about the necessity for aid to reach people, Griffiths tweeted.

Sudan's warring sides have announced multiple truces but none has effectively taken hold.

UN rights commissioner Volker Turk described the situation as "heartbreaking" and "catastrophic". He pointed to a military airstrike in the vicinity of a hospital and the RSF using civilian buildings as bases.

Multiple hospitals have been struck during the war, and the UN says only 16% of Khartoum's hospitals remain fully functional.

Despite the truce efforts, witnesses reported warplanes over north Khartoum and fierce clashes near the state broadcaster's headquarters in the capital's twin city of Omdurman.

Read more: Sudan FM confirms parties will talk only via mediators

Nearly 450,000 fled their homes

According to the International Organization for Migration (IOM), nearly 450,000 civilians have fled their homes, including more than 115,000 who have sought refuge in neighboring countries.

The failure of the warring generals to abide by commitments in efforts to end nearly three weeks of conflict has drawn mounting international criticism.

Egyptian President Abdel Fattah El-Sisi warned on Tuesday that the fighting was affecting "the entire region."

Griffiths also called for the lifting of "bureaucratic impediments to delivering assistance," noting that even he had had difficulty obtaining visas for his trip.

In addition to the capital Khartoum, violence has engulfed the Darfur region, where at least 99 people have been killed, according to Sudan's Doctors Union.

Of the more than 330,000 people displaced inside Sudan, most are from West and South Darfur states, the IOM pointed out.

Toby Harward, who was a UN refugee agency official in Darfur, noted that "there was very widespread damage done to many of the major cities" in Darfur.

Read more: Sudan casualties increase, army accuses regional actors of backing RSF