UN Security Council envoys warned South Sudan's warring leaders they would face sanctions if a civil war that has pushed the young nation to the brink of famine does not stop.
"The council has made it very clear: that it is prepared to impose consequences if there continue to be spoilers, if there continue to be people carrying out gross violations of human rights," US ambassador to the UN Samantha Power said after meeting with President Salva Kiir.
Representatives of the 15-member council, who were in the capital Juba on a two-day mission, were due also to meet rebel chief Riek Machar.
"We will not tolerate violation of the cessation of hostilities and people who spoil the peace agreement," Power told reporters, in one of the strongest warnings yet.
"We have delivered that message here, we will deliver it to Riek Machar," she added.
EU Special Representative to the Horn of Africa, Alex Rondos, also expressed concern "that both sides in South Sudan continue spending their money on arms and fighting for power while South Sudanese citizens are beginning to starve to death".
There is no justification for this humanitarian disaster, he said.
Thousands of people have been killed and more than 1.5 million have fled almost eight months of carnage sparked by a power struggle between Kiir and his sacked deputy Machar, with battles between government troops, mutinous soldiers and ragtag militia forces divided by tribe.
"The parties must know that the people of South Sudan have suffered enough," Rwandan ambassador Eugene-Richard Gasana told reporters.
"The international community will not look on as a seemingly endless situation goes on."
The ambassadors later left to visit the northern town of Malakal, one of the hardest hit areas in the fighting.
The town has been left in ruins after swapping hands several times between government and rebels.
Foreign Minister Barnaba Marial Benjamin told reporters the diplomats would be "shown what South Sudan is doing with regard to peace".
Famine 'now looms'
Power said there were "very worrying reports" more weapons and arms were being brought into South Sudan for a fresh offensive, warning there was "no military solution" to end the conflict.
"There is a grave risk of famine that now looms, that hangs over this visit," she added.
"Fifty thousand children under five are at risk of dying by malnutrition in the coming months, and around half of this country's population is facing grave food insecurity."
The United Nations has said the South Sudan food crisis is the "worst in the world", with aid workers warning of famine within weeks if conflict continues.
The United States said it would provide $180 million in additional aid to help feed people.
The funds would raise to $636 million the total amount Washington has put up in humanitarian assistance.
"But the scale of the suffering and humanitarian need there is shocking, and the threat of famine is real -- so much so that we are using this emergency funding authority for the first time since 2008," the White House said.
US Secretary of State John Kerry on Monday accused both sides of failing to commit to the peace process, a day after they missed a key deadline to forge a unity government.
"Deadlines keep passing and innocent people keep dying," Kerry said.
Stop-start peace talks in the Ethiopian capital Addis Ababa which began in January officially restarted again last week, but the delegates have made little if any progress.
"This is an outrage and an insult to the people of South Sudan," Kerry said. "Their leaders are letting them down again and again."
The United States and the European Union have already imposed penalties on three senior army commanders from the government and opposition, while the regional IGAD bloc have suggested they could follow suit if progress was not made.
Aid agencies have condemned what they say is a determination by leaders to try to defeat the other militarily.
"With the peace talks in Addis Ababa stalling, continuing violence throughout the country and a man-made food security crisis, the situation for South Sudan could not be more urgent," Oxfam's Tariq Riebl said in a statement.
Human Rights Watch has called for an arms embargo and sanctions in an open letter to the Security Council, reporting "extraordinary acts of cruelty that amount to war crimes".