The U.N. refugee agency says thousands of people are believed to be fleeing to Ethiopia to escape fighting in Sudan's Blue Nile state.
U.N. High Commissioner for Refugees Antonio Guterres appealed Saturday for an immediate end to the clashes.
His agency says it has received reports of some 16,000 people fleeing across the border since fighting erupted Thursday.
A UNHCR team was sent to western Ethiopia to assess the situation.
Sudan's president declared a state of emergency in the region on Friday and fired Blue Nile's governor after clashes broke out between armed forces and a rebel group.