Two US military aircraft flying in to evacuate American citizens were fired on and hit over remote Jonglei state, scene of some of the worst clashes in almost a week of fighting in Africa's newest nation.
Four US service personnel were wounded, with one reported to be in a critical condition, before the aircraft turned and headed to Entebbe airport near the Ugandan capital, Kampala. From there the wounded were flown on to Nairobi in Kenya for medical treatment.
The planes came under fire near Bor, which is occupied by forces loyal to former vice-president Riek Machar, an ethnic Nuer. He was sacked in July by South Sudan's President Salva Kiir, who is from the rival Dinka ethnic group.
Last week the rivalry between the two leaders reached a new pitch when Kiir accused Machar of trying to seize power through a coup d'état. It was just a few months before the sacking of Riek Machar that I last visited South Sudan. Even then, the persistent talk among the many South Sudanese and diplomatic officials I met was of an impending coup. Over the last few weeks that threat may have become manifest.
The accused - Riek Machar - denies any attempted overthrow, and Oxford-based South Sudan expert Douglas H Johnson said: "I think all of the bits of information coming out don't point to a coup. I am not even sure we can say there was a concerted mutiny."
But coup or confusion, the incident has revealed just how fragile the coalitions that once held South Sudan together really were, and tensions have quickly boiled over into violence.
United Nation staff say that already hundreds of people have been killed in clashes that spread from the capital Juba and have reached vital oilfields.
It is deepening the most serious internal crisis since the state - roughly the size of France - won independence from Sudan two years ago.
The UN workers say 35,000 civilians are sheltering at their bases.
This weekend the South Sudanese army is trying to retake Bor, the capital of eastern Jonglei state.
Troops backed by helicopter gunships were advancing on the town 90 miles north of Juba, an army spokesman said.
Meanwhile, in Unity state, a major oil-producing region, a senior commander, General James Koang, is reported to have defected to Machar's forces. A resident in Unity state said General Koang announced on local radio he had joined Machar's rebellion.
Machar said General Koang was now in control of the state, but the military, the Sudan People's Liberation Army (SPLA), says he defected alone and did not take any forces with him.
Many South Sudan watchers feel the current crisis is the result of two different processes.
The first is the "state of dissatisfaction" within the governing Sudan People's Liberation Movement (SPLM) party. The second is the integration of militiamen formerly linked to the old enemy in Khartoum into the national army.
"Many of these soldiers do not feel secure," says Johnson.
Earlier this month, a group of disgruntled members of the ruling SPLM party gathered for a press conference in Juba. They kept the location and participants secret until hours before.
The group included Machar, former Cabinet Affairs Minister Deng Alor - now one of 10 senior politicians arrested - and Rebecca Nyandeng.
Nyandeng's presence was particularly notable because she is the widow of John Garang, South Sudan's great martyr, who led southern rebels during the decades-long war with Sudan only to die in a helicopter crash months after signing a peace deal with Khartoum.
It was during this conference that Machar read the group's statement, in which he accused Kiir of exacerbating divisions within the ruling party, cutting himself off from SPLM members and general "dictatorial tendencies". Shortly after this the current fighting started.
Yesterday, Juba was relatively calm, but as the fighting has moved out of the capital, it is increasingly driven by ethnic loyalties as much as political rivalries.
It has also spread to oilfields vital to the impoverished new state's economy and dependent on foreign staff. China National Petroleum Corp, one of the main operators, said it was flying 32 workers out of one field to Juba.
"It's difficult to find plane providers to fly to some of these remote air strips," said one security analyst, who did not wish to be identified.
"The situation on the ground is very fluid and we can't be absolutely confident about exact rebel locations and which airfields they may be controlling."
Just as it is not precisely clear what started the fighting, it is also unclear how it might play out, especially if more people come to perceive it as an ethnic fight. An RAF plane has already picked up 182 Britons from the airport at Juba.
In anticipation of this worst-case scenario, more countries are likely to try to get their citizens out in the coming days.