South Sudan’s army clashed Sunday with ethnic Nuer fighters, and government officials accused rebels of mobilizing the force despite the government’s offer of a truce to end the conflict.

Information Minister Michael Makuei said the Nuer militia had dwindled in numbers — from an estimated 25,000 — after Nuer politicians and tribal elders persuaded it to abandon its march on Bor, the provincial capital of Jonglei state.

“About 5,000 refused to abandon the march, and they have proceeded with their advance on Bor. They then dislodged [government troops] from Mathiang, about 18 miles from Bor,” Makuei said by phone from South Sudan’s capital, Juba, 120 miles south of Bor.

Government troops retook Bor from rebel forces Tuesday after several days of heavy fighting. Witnesses spoke of panicked civilians fleeing the city to escape another round of bloodletting.

Nuer fighters — called the “White Army” because they dust their bodies with ash to ward off insect bites — have threatened the central government in the recent past.

Former vice president Riek Machar, whom President Salva Kiir has accused of a coup attempt, is an ethnic Nuer.

Bor was the scene of a massacre of ethnic Dinka in 1991 by Nuer fighters loyal to Machar.

The United Nations said the involvement of the Nuer fighters brings another volatile ingredient to the situation.

“South Sudan does not need another escalation of the crisis involving armed youth, pitching communities against communities. This can end in a vicious cycle of violence,” Hilde Johnson, special representative of the U.N. secretary general, said in a statement.