“They came to my house,” the man, Biel Boutros Biel, a human rights activist, said of the South Sudanese security forces. “I knew they were coming after me.”

After President Salva Kiir announced that his government had headed off a coup attempt by his former vice president last week, South Sudan was tossed into uncertainty and upheaval. Hundreds are believed to have been killed in the capital, Juba, with thousands more fleeing into the bush to escape the violence.
The former vice president, Riek Machar, has denied any involvement in a coup attempt, accusing the president of using the unrest as a pretext to crack down on his opponents. But while both sides debate what ignited the fighting, the aftermath has taken on a life of its own, revealing longstanding tensions in a fledgling new nation that international officials worry could ultimately tear it apart.
“Today, that future is at risk,” President Obama said in a statement on Thursday. “South Sudan stands at the precipice. Recent fighting threatens to plunge South Sudan back into the dark days of its past.”
Events are unfolding fast. United States aircraft sent to evacuate Americans came under fire on Saturday, wounding four soldiers. Rebels are believed to have taken control of some of the country’s oil fields and alliances are shifting.
Like many conflicts in this tenuous nation, the fighting has taken on an ethnic dimension, human rights workers say. Mr. Machar, the former vice president removed over the summer when Mr. Kiir summarily dismissed his entire cabinet, is a Nuer. The president belongs to the majority Dinka ethnic group.
In the capital, South Sudanese forces have targeted members of the Nuer ethnic group, killing many and detaining others, including soldiers, lawmakers and students, rights workers and refuge seekers say.
But outside the capital, in Jonglei State, the reverse has occurred as well, with Nuer militiamen targeting Dinka, descending on United Nations compounds where thousands of civilians have fled for safety and carrying out attacks on oil facilities that have resulted in what the Security Council called “the heavy loss of life” among workers.
“We are deeply concerned that ethnically based attacks on all sides will lead to revenge attacks and more violence,” Daniel Bekele, Africa director at Human Rights Watch, said in a statement.
Other observers have said that politics, not ethnicity, are driving the conflict.
“It is a power struggle,” said Zacharia Diing Akol, an analyst at the Sudd Institute in Juba. “Ethnicity is an afterthought.”
South Sudan became independent in 2011 when it broke away from its neighbor to the north, Sudan, after decades of civil war. Now, critics accuse South Sudan’s president, Mr. Kiir, of being an autocrat who oversees a government marred by corruption, mismanagement and a lack of freedom.
Opponents of Mr. Machar, on the other hand, see him as an opportunist who changed sides during the civil war against Sudan to gain advantages for himself and his fellow Nuer. Mr. Machar, a British-educated former rebel with a Ph.D., was a senior member of the Sudanese People’s Liberation Army, the military wing of the party Mr. Kiir now leads, as it battled Sudan’s government in Khartoum. He split from the movement in 1991 and formed his own group, which signed a peace agreement with the Sudanese government in 1997.
During this period, Mr. Machar’s group fought against the South Sudanese rebels. But he later defected from Khartoum and rejoined the Southern rebel forces. When South Sudan seceded in 2011, Mr. Machar was made vice president until Mr. Kiir fired him along with the entire cabinet in July.
The United Nations said that the upheaval in recent days had forced 20,000 people to seek refuge at its compound in Juba, raising humanitarian concerns.
“There is no food, no water, and medical services are limited,” said Mr. Biel, the rights activist. “This is not a place to be.”
Diplomats have responded with great concern. On Friday, the United Nations Security Council issued a statement expressing “grave alarm and concern regarding the rapidly deteriorating security and humanitarian crisis in South Sudan resulting from the political dispute among the country’s political leaders, which threatens serious implications for the long-term security and stability of South Sudan, as well as for the neighboring countries and other peace and security challenges in the region.”
There are fears that the growing instability will have an impact on an already delicate economy. About 200 oil workers in Unity State to the north have sought refuge at a United Nations base there, and the Chinese oil company operating there, CNPC, has begun removing its workers.
A descent into civil war in South Sudan would have serious local and regional consequences.
“In the coming days, refugees are likely to reach Uganda, Kenya and Ethiopia,” said Casie Copeland of the International Crisis Group. “South Sudan has become a critical player in the broader East African economy, with substantial regional and international investment.”
Many groups have called for greater international involvement. “The international community needs to double down on diplomatic engagement to facilitate a political resolution to the crisis,” said Akshaya Kumar, Sudan and South Sudan policy analyst at the Enough Project.
But at the United Nations compound in Juba, Mr. Biel remained skeptical about a quick resolution. “If I was at home, I would not be alive,” he said. “The whole nation is now suffering.”
By ISMA’IL KUSHKUSH