United Nations Secretary General Ban Ki-moon says he is deeply concerned about the deadly attacks in southern Sudan's oil-rich Upper Nile region that have killed 25 people and displaced dozens more.

Mr. Ban said in a statement Sunday that the South Sudanese government should "re-double its efforts to ensure the protection of civilians" and work with the U.N. to resolve the root causes of the violence.

Military officials in southern Sudan said the clashes began Friday when heavily armed fighters attacked an ethnic Dinka village in Bony-Thiang, killing 20 civilians.  The officials said Dinka villagers retaliated Saturday by attacking a Shilluk tribal settlement nearby, killing five people.

Also Sunday, the World Health Organization warned that another part of southern Sudan is at high risk for epidemics because of attacks by Ugandan rebels.

The agency said the health care system in Ezo County (Sudan) has been disrupted by repeated attacks in the area from the rebel Lord's Resistance Army.

A WHO spokesman in Geneva, Paul Garwood, said the LRA pillaged health care facilities in August and made it impossible for doctors to carry out an immunization program.  He said this has left many young children susceptible to measles, polio and other life-threatening diseases.

The United Nations says recent LRA attacks in Ezo County have displaced thousands of people, including many health care workers.  Ezo borders the Democratic Republic of Congo and the Central African Republic.

Attempts by the Ugandan, Congolese and southern Sudanese governments to shut down the LRA have been largely unsuccessful.  The group continues to operate out of the northeastern Democratic Republic of Congo and sometimes crosses into neighboring countries.