The United States on Tuesday threatened to take unspecified measures against South Sudan's government unless it moves to end the nearly four-year war and stop harassing UN peacekeepers and aid workers.
US Ambassador to the UN Nikki Haley put the onus squarely on President Salva Kiir to take action, telling the Security Council that "words are no longer sufficient."
"The United States is prepared to pursue additional measures against the government - or any party, for that matter - if they do not act to end the violence and ease the suffering in South Sudan," Haley said.
She did not provide details of the measures, but the United States unsuccessfully pushed last year for an arms embargo on South Sudan and international sanctions on senior officials.
"Going forward the United States will judge President Kiir and his government by their actions, not their words," she said.
Kiir must uphold ceasefires, join a new regional peace initiative, stop placing restrictions on UN peacekeepers and allow access for aid groups.
A report released on Tuesday by UN sanctions monitors accused the government of using food aid as a weapon of war during its campaign against opposition rebels in the northwestern city of Wau.
After aid finally reached civilians in August, the first time in a year, humanitarian workers "witnessed significantly high levels of malnutrition, with high rates of severe acute malnutrition," the report said.
Between January and September, 164 young children and elderly persons died from hunger and disease in that area, it said.
Haley travelled to South Sudan in October and held talks with Kiir, becoming the highest level US administration official to visit Juba.
The United States is South Sudan's biggest aid provider and a key supporter of its 2011 independence from Sudan.
South Sudan descended into war in December 2013 when Kiir accused Riek Machar, his former deputy, of plotting a coup.
Tens of thousands have died in the fighting and nearly four million have been driven from their homes.