Gunmen 'armed' by Khartoum kill 40, says 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 risks 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 mutual 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.

Oil-producing Unity state is a base for a number of rebel groups that Juba claims are backed by Khartoum to destabilize South Sudan 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 added.

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

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

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