"UNHCR, with governments and partners, is preparing for the possibility that over 800,000 people may flee the fighting in Sudan for neighboring countries," the agency's chief Filippo Grandi wrote on Twitter.
He added that the UNHCR's planning figure does not mean it expects this number of people to flee but is laying plans to meet the needs that could arise.
The fighting, which erupted on April 15 between Sudan's army chief and a paramilitary force commander, has already led to the deaths of more than 500 people and triggered a mass exodus of refugees and foreigners.
The latest estimate by the UNHCR marks a dramatic increase from last Tuesday, when it gave a planning figure of possibly 270,000 people fleeing into neighboring Chad and South Sudan. (ILKHA)