Children displaced by recent fighting walk through flood water inside the United Nations Mission in South Sudan camp in Malakal, Upper Nile State, on Thursday. (Staff/Reuters)

Ethnic violence in South Sudan is spiraling toward all-out genocide, Secretary of State John F. Kerry said Thursday in the strongest American warning yet that the African nation the United States helped to establish three years ago is at risk of collapse.

International peacekeepers with a U.N. mandate must get on the ground fast to separate the warring factions, Kerry said. He warned of “widespread famine” among the roughly 1 million people displaced by fighting, and he threatened U.S. and other economic sanctions, as well as possible war crimes prosecutions, if the killing of civilians goes on.

“With respect to the question of genocide, there are very disturbing leading indicators of the kind of ethnic, tribal, targeted nationalistic killings taking place that raise serious questions,” he said after meetings with diplomats leading peace talks, which have so far proved fruitless.

Personal and ethnic rivalries between South Sudanese President Salva Kiir and his former deputy, Riek Machar, erupted into warfare in December. The pace and severity of attacks has escalated in recent weeks, raising the specter of a genocidal rampage like the one in nearby Rwanda two decades ago.

If the tit-for-tat killings “continue in the way that they have been going, [they] could really present a very serious challenge to the international community with respect to the question of genocide,” Kerry said, stepping carefully around a legal definition that would compel the United States and others to act.

At a hospital for prevention of mother to child transmission of HIV/AIDS in Addis Ababa, Sec. of State John Kerry said the U.S. was working with Ethiopia to improve health facilities. (Reuters)

“It is our hope that that could be avoided,” he told reporters here in the Ethiopian capital, where leaders of regional powers have helped renew peace talks between the factions this week. “It is our hope that in these next days, literally, we can move more rapidly to put people on the ground who can begin to make a difference.”

South Sudan voted in 2011 for independence from Sudan, after decades of civil war and a fragile peace brokered partly by Washington. The United States was instrumental in getting the new government running, and it has pumped tens of millions into diplomatic and economic projects in South Sudan. Kiir has carried the official U.S. stamp of approval despite reservations within the administration and in Congress about his ethnic ambitions and ability to govern.

Kerry said the United States and East African nations are committed to deploying a predominantly African military force in South Sudan. He gave no details on the force, which would supplement about 9,000 U.N. peacekeepers, but said he and regional foreign ministers had agreed Thursday on its outlines.

Ethiopian Foreign Minister Tedros Adhanom Ghebreyesus emerged from a meeting with Kerry and diplomats from Kenya and Uganda to say that the task is urgent.

“There is an agreement now that we have to really be as aggressive as possible in order to have impact on the ground in South Sudan,” he said.

Thousands of people have been killed since the political rupture between Kiir and Machar in December. Kiir is an ethnic Dinka, and Machar is a Nuer. U.S. officials say the two men are stoking ethnic tensions to settle personal scores. A January cease-fire agreement never took hold. Last month, civilians were massacred in mosques and targeted in hospitals.

The stalled peace talks got a potential boost with the Kiir government’s release of several detainees, as demanded by Machar’s camp. But a U.N. envoy emerged from discussions with Machar this week with no announcement of a breakthrough.

The Obama administration has authorized travel bans and the seizure of overseas assets for those found culpable in the violence, but it has not finalized a list of intended targets, a senior State Department official traveling with Kerry said. The official spoke on the condition of anonymity to preview Kerry’s diplomatic outreach.

“Those who are responsible for targeted killings based on ethnicity or nationality have to be brought to justice,” Kerry said. “We are actively considering sanctions against those who commit human rights violations and obstruct humanitarian assistance.”

The Obama administration has not ruled out that Kiir and Machar could be among them, and Kerry hinted strongly that they will be.

“The place to start is the place where it started,” he said of impending sanctions. “And that is with the former vice president and with the current president of South Sudan.”

International war crimes prosecutions, if any, would come much later.

Kerry blamed both sides, but he leveled his harshest criticism at Machar.

“The current president of South Sudan is the elected, constitutional president of a country, and Mr. Machar is a rebel who is trying to unconstitutionally take power through force,” Kerry said. “There’s no equivalence between the two as far as we are concerned.”