A U.N. official says more than 23,500 people have fled fighting in South Sudan's Jonglei state and sought refuge in neighboring countries of Uganda, Kenya and Ethiopia.

Tim Irwin, a spokesman in South Sudan for the U.N.'s refugee agency said Friday the majority of the refugees were fleeing to Ethiopia.

Irwin says that since the beginning of May about 16,000 people have arrived in Ethiopia, 5,000 in Kenya and 2,500 in Uganda.

The most recent fighting started two months ago after South Sudan's government launched an operation to crush 10 months of rebellion led by David Yau Yau, a former colonel in South Sudan's military.

The U.N. says that the thousands of civilians displaced from their homes in Jonglei state's Pibor County had no access to humanitarian services.