US Ambassador Nikki Haley told the Security Council that President Salva Kiir's government was "increasingly proving itself to be an unfit partner" in efforts to restore peace and urged African leaders to hold it accountable.
"The time has come to acknowledge the hard reality - that the leaders of South Sudan are not just failing their people, they are betraying them," Haley told a council meeting on South Sudan.
South Sudan's food crisis is worsening with 5.1 million people severely food insecure of whom 1.5 million are just one step away from famine, UN aid official Ursula Mueller told the council.
Hours after the December 24 ceasefire went into effect, South Sudan's government and main rebel group accused each other of launching attacks, dealing a blow to a new regional peace effort.
The Security Council has thrown its weight behind the new peace drive led by the regional IGAD grouping, warning that this effort represented a "last chance" for peace.