United Nations Secretary General Ban Ki-moon has called on the leaders of Sudan and South Sudan to settle an escalating row over oil.
At the opening of the African Union summit in the Ethiopian capital Addis Ababa, Ethiopia, Mr Ban said the dispute had become a major threat to stability in the region.
He said there was "great concern" that war could break out again.
All oil production in South Sudan is halted and no oil is flowing through Sudan for export after talks on transit fees broke down.
Sudan earlier seized tankers carrying southern oil in lieu of payments, and oil minister Stephen Dhieu Dau said on 20 January that oil production would be halted.
The BBC reports revenue from oil comprises 98% of South Sudan's budget.
The government in South Sudan has accused Khartoum of stealing oil worth $US815 million in total.