The Obama administration on Monday expressed concern about a coup attempt in South Sudan, calling on all parties to resolve their differences through peaceful means. "We call on all parties to resolve their differences through peaceful democratic means, not through the use of violence," State Department spokeswoman Marie Harf told reporters at a regular news briefing. South Sudan's President Salva Kiir said Monday that an attempted coup launched overnight by soldiers loyal to his former deputy Riek Machar had been put down, and that "the situation has completely been controlled and the attempt failed." "It's a very fluid situation," Harf said. "We're very closely reviewing the security situation in South Sudan." In his statement, Kiir also declared a state of emergency in the country and imposed a curfew from 6:00 p.m. to 6:00 a.m. local time. Deep differences had emerged between two groups within the ruling Sudan People's Liberation Movement after Kiir removed Machar from office in July. The youngest country on earth, South Sudan won independence from Sudan in July 2011.