UNITED NATIONS (AP) — U.S. Ambassador Nikki Haley warned South Sudan's government and rival forces Tuesday that the United States is ready to pursue additional measures if they don't take action to end violence, start negotiations, and ease the humanitarian crisis in the conflict-wracked country.

Haley told the U.N. Security Council that "words are no longer sufficient" and it's time for action now, especially by President Salva Kiir.

She said the government bears primary responsibility "for the killing, raping, and torturing in South Sudan" — and for ending the four-year conflict and "saving future generations of South Sudanese."

There were high hopes that South Sudan would have peace and stability after its independence from neighboring Sudan in 2011. But the world's newest nation plunged into ethnic violence in December 2013 when forces loyal to Kiir, a Dinka, started battling those loyal to his former vice-president Riek Machar, a Nuer who now lives in South Africa.

An August 2015 peace agreement has not stopped the fighting, and clashes in July 2016 between supporters of Kiir and Machar set off further violence. The conflict has killed tens of thousands of people.

Haley said the way to reinvigorate the peace process is for government and opposition leaders to seize the opportunity of the revitalization process put forward by the eight-nation East African regional group known as IGAD.

"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," she said. "That means putting down their weapons, coming to the negotiating table through the High-Level Revitalization Forum, and ending the harassment and obstruction" of U.N. peacekeepers and others trying to help people in desperate need.

U.N. envoy for South Sudan, Bintou Keita, called the security situation in South Sudan "precarious."

She expressed serious concern at "the real risk of an escalation in violence with the onset of the dry season, and the government's push to assert military dominance across the country, notably when faced with continued resistance by armed groups."

Keita said the humanitarian situation in South Sudan remains "dire," and is compounded by widespread conflict, inter-communal violence, and restrictions on access to thousands of people who need assistance.

Both Keita and Haley said they were encouraged at Kiir's new order on Nov. 9 calling for unhindered humanitarian access — but are waiting to see it implemented at all levels.

"The catastrophic consequences of the humanitarian crisis are borne out by shocking statistics: four million South Sudanese are either internally displaced or have sought refuge in neighboring countries; an estimated six million people (half of the population) are severely food insecure including 1.7 million on the brink of famine," she said.

Keita said preparations for the IGAD forum are entering the final phase and the regional group reports that all groups consulted so far have stated their "full and unconditional support" and readiness to engage.