Haley said yesterday Peter Alex, a South Sudanese who works for the World Food Program, had been reunited with his family.
South Sudan is "the most dangerous country in the world today for aid workers," the ambassador said in a statement. Last week, she said 83 humanitarian workers had been killed since the outbreak of civil war in December 2013.
WFP spokesman Steve Taravella confirmed the release of Alex, saying he had been detained on April 13 in the town of Wau while "carrying out his official duties" and was released without charge on Tuesday. It was not clear why Alex had been detained.
South Sudan's government and opposition forces have repeatedly targeted aid workers who are also trying to help hundreds of thousands of people in two counties where a famine has been declared. Taravella said WFP is concerned that "the lack of security for our staff in the region is severely limiting our ability to assist people in need."
Haley urged combatants to stop the violence and allow the delivery of aid "to address the humanitarian crisis that the actions of the warring parties have created for their own people."