The U.N. Mission in South Sudan said at least two dozen members of the South Sudanese military were killed last week by armed men loyal to militia leader David Yau Yau.
A military spokesman told Voice of America last week that the rebel leader was recruiting children into his militia's ranks. He was a member of the military but broke away in April.
UNMISS, in a statement, said the rise in violence in Jonglei state, near the Ethiopian border, was a threat to South Sudan's national security.
"It is essential to preserve the gains achieved over recent months in the improved security situation in Jonglei and ensure the civilian disarmament process goes forward as a key element of this effort," the mission said.
A U.N. report in June said violence between rival ethnic groups in Jonglei state led to deaths, abductions of women and children, destruction of homes and the displacement of thousands of people.
The report described a "cycle of violence" in the region.
South Sudan gained independence last year under the terms of a peace agreement with Khartoum. Oil disputes, border conflicts and ethnic tensions are threatening the deal, however.