Since April 15, the country has turned into the scene of incessant violence between the army and paramilitary Rapid Support Forces (RSF). More than 500 people have been killed, while hospitals and other services have been knocked out and residential areas turned into war zones.
"It has been more than two weeks of devastating fighting in Sudan, a conflict that is turning Sudan humanitarian crisis into a full-blown catastrophe," Abdou Dieng, told a briefing via video link on Monday.
He also warned about the repercussions that the unrest could have for Sudan's neighbors, saying, "The regional spillover effect of the crisis is a serious concern."
The comments came as the prospect of negotiations between the warring sides remained bleak.
Army chief, General Abdel Fattah al-Burhan, has ruled out sitting down for talks with the head of the RSF, General Mohamed Hamdan Dagalo, who, in turn, has said he would talk only after the army has ceased hostilities.
A shaky ceasefire between the army and the RSF was extended for another 72 hours at midnight on Sunday, amid a blame game between the two sides, each accusing the other of violating the truce.
Similar to the previous such deals, the truce was soon followed by reports of fresh clashes between the two sides.
UN: More than 800,000 may flee Sudan
Also on Monday, another UN official said more than 800,000 people might flee Sudan as a result of the fighting.
"Without a quick resolution of this crisis, we will continue to see more people forced to flee in search of safety and basic assistance," UNHCR's Assistant High Commissioner for Operations Raouf Mazou told a briefing in Geneva.
"In consultation with all concerned governments and partners, we've arrived at a planning figure of 815,000 people that may flee into the seven neighboring countries," he added.
So far, he said, some 73,000 people have fled to Sudan's neighbors, including South Sudan, Chad, Egypt, Eritrea, Ethiopia, the Central African Republic, and Libya.