The UN Security Council on Thursday gave South Sudan's warring sides a month to reach a peace deal or face possible sanctions.
A resolution drafted by the United States narrowly won backing in the 15-member council, picking up nine votes.
Six countries abstained, including Russia, China and Ethiopia, a key player in regional peace efforts.
The resolution requires UN Secretary-General Antonio Guterres to report by June 30 on whether a ceasefire agreed in December — the latest in a string of truce deals — was holding and whether the sides have "come to a viable political agreement."
If not, the council "shall consider" within five days of the report slapping sanctions on South Sudan's defence minister and five other officials and possibly imposing an arms embargo.
South Sudan won independence from Sudan in 2011, with critical backing from the United States, which remains Juba's biggest aid donor.
But the US administration has grown increasingly frustrated with President Salva Kiir's government as the brutal war grinds on, now in its fourth year.
"The United States has lost its patience," US Ambassador Nikki Haley told the council ahead of the vote.
"The status quo is unacceptable. It is long past time for all of us to demand better for the South Sudanese people."
Ethiopia and Equatorial Guinea argued that peace efforts must be given more time, but Ivory Coast - the third African country on the council - backed the resolution.
Ethiopia's Ambassador Tekeda Alemu warned that the measure could lead to the collapse of the regional peace effort by the eight-country Intergovernmental Authority on Development (IGAD).
"Yes, we in the region are also extremely frustrated," Alemu said, but he added that the sanctions resolution "will be detrimental to the process."
Ethiopian-led peace efforts have yet to yield a breakthrough to end the war that began in December 2013 when Kiir accused his former deputy Riek Machar of plotting a coup.
The United States has put forward a sanctions blacklist of six officials but the council would likely have to give final approval to impose a global travel ban and assets freeze on them.
The proposed sanctions blacklist would target Defence Minister Kuol Manyang Juuk for leading attacks on the northeastern town of Pagak that was captured from rebel forces in 2017.
Also listed is cabinet minister Martin Elia Lomuro for threatening the press, obstructing humanitarian aid and impeding the work of the UN peacekeeping mission in South Sudan.
Information Minister Michael Makuei is cited for his role in planning an attack in 2014 on a United Nations compound in Bor and overseeing a campaign to suppress the media.
Former military chief Paul Malong faces possible sanctions for ordering government forces to attack civilians, schools and hospitals and deputy army chief of staff Malek Reuben for overseeing an offensive in 2015.
Also included is Koang Rambang Chol for leading attacks in northern Bieh state and ordering his forces to impede the work of aid workers.
Since the start of the war in December 2013, nearly 1.8 million people have been forced to flee their homes and tens of thousands have been killed.