JUBA. South Sudan sent troops to secure a United Nations base after armed civilians fired on displaced tribespeople sheltering there, in an attack that killed at least 48, the president’s spokesman said on Friday.

Locals pretending to be peaceful protesters delivering a petition forced their way into the camp on Thursday and opened fire before being beaten back by UN security personnel (UNMISS).

"The army has come in now. They have been ordered to protect UNMISS so there will be no attack from anybody,” Ateny Wek Ateny, President Salva Kiir’s spokesman, told Reuters by phone.

Thousands of people have been killed and more than one million displaced since fighting erupted in South Sudan in the middle of December, triggered by a power struggle between Kiir and former Vice President Riek Machar.

The conflict in Africa’s newest state took on a tribal dimension as Kiir’s Dinka fought Machar’s Nuer for control of strategic towns before a ceasefire was signed on January 23.

Sporadic clashes between both sides after the ceasefire deal erupted into full-blown combat this week, when the rebels seized control of Bentiu, the capital of oil-producing Unity state.

Thursday’s attack on the U.N. base at Bor, some 120 miles north of the capital of Juba, was blamed on locals who were seeking to punish the Nuer for the loss of Bentiu.

"Those internally displaced people in Bor from the Nuer community were celebrating the capture of Bentiu by the rebels and this angered the local community,” Ateny said.

The Dinkas are the predominant group in the area.

The locals went to the base to demand the relocation of the 5,000 Nuer living there and were dispersed by UN personnel before regrouping nearby and launching the attack, he said.

Joe Contreras, the acting spokesman for UNMISS, said security had been stepped up in their bases around the country - where tens of thousands are sheltering - and urged South Sudan to investigate the attack and prosecute the assailants.

No one has been arrested over the attack pending completion of investigations, information minister Michael Makuei told Reuters.

The conflict has disrupted oil production, which provides most government revenue. The rebels warned oil firms to pack up and leave within a week after they recaptured Bentiu on Tuesday.