At least 528 people have been killed and nearly 4,600 wounded, according to the latest health ministry figures, which are likely to be incomplete.
The visit by top U.N. humanitarian official Martin Griffiths comes one day after neighboring South Sudan announced that the warring sides had agreed "in principle" to a seven-day ceasefire.
"Just arrived in Port Sudan to reaffirm the UN's commitment to the Sudanese people," he said on Twitter.
Griffiths had earlier called for civilians and aid workers to be protected.
"Ensure safe passage for civilians fleeing areas of hostilities. Respect humanitarian workers and assets," he said on Twitter.
The United Nations called Wednesday for security guarantees at "the highest level" to ensure desperately-needed aid deliveries in conflict-torn Sudan, after six trucks carrying food aid to the Darfur region were looted.
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.
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 Dagalo, who commands the paramilitary Rapid Support Forces (RSF).
More than 430,000 civilians have fled their homes, the United Nations said, including 100,000 who have sought refuge in neighboring countries.
Only 16 percent of Khartoum's hospitals remain fully functional, the UN added.
The foreign ministry of neighboring South Sudan have announced that al-Burhan and Dagalo "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 two men have agreed multiple truces since the fighting began 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.
Despite the truce efforts, witnesses reported warplanes over north Khartoum on Wednesday and fierce clashes near the state broadcaster's headquarters in the capital's twin city of Omdurman.
"We heard again loud gunfire and anti-aircraft firing at a fighter jet this morning," a resident of south Khartoum said.
Lawlessness has also engulfed the Darfur region, with at least 99 people killed in fighting, according to Sudan's doctors union.
Of the more than 330,000 people displaced inside Sudan, over 70 percent were reported to be from West and South Darfur states, according to the International Organization for Migration.
Darfur is still scarred by a war that erupted in 2003 when then-strongman Omar al-Bashir unleashed the Janjaweed militia, mainly recruited from Arab pastoralist tribes, against ethnic minority rebels.
The failure of the warring generals to abide by their commitments in efforts to end nearly three weeks of fighting has drawn mounting international criticism.
"The two generals, even though they accept the ceasefire, at the same time they continue fighting and shelling the city," complained Ismail Wais of East African regional bloc IGAD.
He said the persistent fighting "compounds and complicates the political, security and humanitarian situation on the ground making it harder to resolve."
Egyptian President Abdel Fattah al-Sisi warned on Tuesday that the fighting in neighboring Sudan was affecting "the entire region."