JUBA, Sudan, (Reuters) – Armed Nuer tribesmen killed at least 139 members of a rival tribe in an attack in a remote area of southern Sudan, an official said on Thursday.

The Nuer tribesmen attacked Dinka cattle herders in Tonj, one of the most remote parts of oil-producing south Sudan, on Saturday and seized about 5,000 animals, the deputy governor of Warrap state, Sabino Makana, told Reuters. “They killed 139 people and wounded 54. Nobody knows how many attackers were killed. But it may be many as a lot of people came to fight.”

A surge of tribal violence in 2009 killed about 2,500 people and forced 350,000 to flee their homes in the south, said a report issued by ten aid groups including Oxfam, Save the Children and TearFund on Thursday.

There was now a risk the violence could escalate, undermining a fragile 2005 peace deal that ended more than two decades of north-south civil war, said the report. “A lethal cocktail of rising violence, chronic poverty and political tensions has left the peace deal on the brink of collapse,” it said.

The underdeveloped region has been plagued by violent tribal clashes, often sparked by cattle-rustling raids.

Southern leaders last year accused Khartoum of backing militias to undermine the south, although some politicians acknowledged southern officials may also have been arming fellow tribesmen to build up support ahead of elections due in April.

The United Nations told Reuters it was sending a team to the Tonj area to check on the reports, saying other sources had confirmed there were a large number of deaths.

The head of the U.N.’s Office for the Coordination of Humanitarian Affairs in the south Lise Grande said fighting had also been reported in three other areas.