Hundreds, possibly thousands, of people have died since Ethiopian government troops launched a war two weeks ago in northern Tigray state against regional authorities they accused of staging a surprise attack on federal forces, Reuters reported.
Thousands of refugees have fled, some crowding into boats to cross a river to Sudan, overwhelming aid groups positioned on the other side.
"Together with all the agencies we have built a response plan for about 20,000 people and currently we are at about 31,000 so it has already surpassed that figure," Axel Bisschop told a Geneva briefing. "The new planning figure is around 200,000."
UNICEF's Sudan-based representative Abdullah Fadil said there were concerns about disease spreading amongst the crowds, of whom nearly half are children.
UN agencies are seeking $50 million in immediate funding which will go towards providing food and setting up new camps.