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JUBA, Sudan (Reuters) - Armed Nuer tribesmen have 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 gunmen attacked Dinka cattle herders on Saturday in Tonj East, one of the most remote parts of oil-producing south Sudan, and seized about 5,000 animals, the deputy governor of the surrounding 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 resulted in the deaths of about 2,500 people and forced 350,000 to flee their homes in the south, a report issued by ten aid groups including Oxfam, Save the Children and TearFund said 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, the report added.

"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 long been plagued by violent tribal clashes, often involving cattle-rustling, although the scale of recent attacks has shocked observers.

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.