South Sudan became an independent country in 2011 under the terms of a 2005 peace deal with Sudan brokered with Washington's help. The peace deal ended Sudan's civil war, though the peace is threatened by border conflicts, rebel fighting and disputes over oil.
Kerry said Wednesday the United States would do "everything" it can to ensure gains secured by the 2005 peace deal endure in the region.
"Khartoum and Juba can choose the path of peace and prosperity, or they can go backwards," he said in a statement.
Kerry made his statement during an announced that Ambassador Don Booth would serve as a joint special envoy for Sudan and South Sudan. Booth last served as the U.S. ambassador to Ethiopia.
"If both states put their people first by governing inclusively and democratically, protecting human rights, and creating economic opportunity, they can make progress on realizing the future their people deserve," Kerry said. "This is a decisive period."
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