The U.N. refugee agency says 25,000 people have fled from Sudan's troubled Blue Nile state into western Ethiopia since September 3, and the number is expected to rise.
The United Nations says the influx is expected to rise because of ongoing fighting in Blue Nile, where Sudan's army is battling local rebels.
In a statement on Tuesday, U.N. agencies and the Ethiopian government formally appealed for $18.3 million to help the refugees.
The agency says many of the refugees are escaping violence around the state capital, Damazin, where aerial bombings were reported on Monday. Spokesman Adrian Edwards told reporters in Geneva on Tuesday that most of the refugees have been entering Ethiopia through the Kurmuk border point.
Last week, a U.S.-based Satellite Sentinel Project said satellite images showed thousands of Sudanese government troops marching toward Kurmuk, a rebel stronghold.
Khartoum has repeatedly accused newly-independent South Sudan of supporting rebels in the border area state. South Sudan denies the charge.
The U.N. refugee agency says an increasing number of Sudanese crossing into Ethiopia includes families who are bringing their belongings, including livestock. Many reported walking for a week to reach safety.