As its independence draws near, South Sudan has yet to agree how to divide oil revenues with its northern neighbor, which has the infrastructure to export the oil the south needs to sell to survive.
South Sudan says the North is at risk of breaking a fragile 2005 peace deal that ended two decades of civil war.
According to South Sudanese leaders, the seizure of the strategic, oil-rich town of Abyei was a declaration of war by the northern government.
Violence in towns along Sudan's north-south border has prompted a flurry of accusations that are setting a poor stage for the country to peacefully split this summer.
As the week-long Sudan referendum vote wraps up, the United States's words and actions show that it intends to continue a high level of diplomatic engagement.