UN General Secretary Ban Ki Moon has called South Sudan's seizure of Sudan's Hij-leej oil field, illegal.
He's also urged both sides to show restraint and withdraw troops from the volatile border region.
The Arab League says it will hold an emergency meeting to discuss the escalating violence there.
Earlier, Sudan's President Omar Al Bashir threatened war against his Southern neighbour.
Al Jazeera's Harriet Martin reports from Khartoum.
