Gunmen 'armed by Khartoum' kill 40 in South Sudan

South Sudan on Monday accused its former foes in the Khartoum government of arming gunmen who killed over 40 people in a cattle raid, as the UN warned tensions between the two sides risk regional peace.

"A militia group from Unity state penetrated into Warrap state... and attacked people in a cattle camp, killing over 40," said Interior Minister Alison Manani Magaya, the latest wave of violence in the world's newest nation.

"This militia group was armed by the government of Khartoum," he added.

South Sudan seceded peacefully from Sudan in July after decades of war, but both countries have since repeatedly exchanged allegations that each side backs proxy rebel forces against the other.

Oil-rich but grossly impoverished South Sudan was left awash with guns after years of conflict, and brutal tit-for-tat raids by rival ethnic groups to steal cattle from each other are common.

United Nations chief Ban Ki-moon said Sunday tensions and a furious row over oil between the former enemies has become a major threat to regional peace and security.

"The situation in Sudan and South Sudan has reached a critical point. It has become a major threat to peace and security across the region," Ban told an African Union summit in the Ethiopian capital.

Key issues unresolved at independence have escalated into bitter arguments, including a row over pipeline transit fees to transport the landlocked South's oil to port in the rump state of Sudan.

Juba said Sunday it had nearly completed a shutdown of its oil production -- the fledgling nation's top revenue source -- after it accused Khartoum of steSaling $815 million of its oil, and AU-mediated talks stalled.

In addition, tensions have been raised by their still undemarcated border, parts of which cut through oil fields.

The South's oil-producing border state of Unity is a base for a number of rebel groups that Juba claims are backed by Khartoum to destabilize the fledgling nation by attacking civilians and laying landmines.

Magaya could not name the specific group responsible for the attack, which took place over the weekend, but claimed that rebel groups in Unity state were collaborating with one another.

"The number of wounded is still not clear, but they took a lot of cattle with them," he said, added that the gunmen were from the Nuer ethnic group, while those attacked were Dinka.

He said government teams had been sent to investigate and that the death toll could rise as local officials "were still counting the bodies."

South Sudan is reeling from an explosion of ethnic violence, notably in Jonglei state, where a militia army of up to 8,000 armed youths attacked a rival ethnic group earlier this month affecting 120,000 people, according to the UN.

The attacks were a dramatic escalation of centuries old tit-for-tat cattle raids, with aid workers reporting horrific killings, including babies beaten against trees and women hacked by machetes.

The UN has warned South Sudan faces massive challenges as the world's newest nation struggles to support hundreds of thousands of fleeing violence.

Last year, over 350,000 people were forced from their homes due to violence, according to UN figures, while since June South Sudan has also taken in over 80,000 refugees fleeing civil war in north Sudan.