After several weeks of relative calm, fighting broke out in South Sudan on Friday, forcing aid workers to take cover in Bentiu, where more than 40,000 civilians have sought safety at a United Nations base. Bentiu has traded hands multiple times since violence spread across South Sudan last December. Aid organizations fear that South Sudan could be facing a famine. Since December, more than one million people have fled their homes, meaning crops were not planted before seasonal rains began. The violence often pits one ethnic group against another in South Sudan. Samantha Power, the American ambassador to the United Nations, said during a United Nations Security Council trip to South Sudan this week that United Nations officials had “very worrying reports” of arms being brought into the country to set the stage for more fighting when the rainy season ends. Ms. Power has called on President Salva Kiir and the rebels’ leader, Riek Machar, to set aside their differences and resolve the crisis.