As many as 500 people are said to have been killed in Juba, the capital of South Sudan, the world’s youngest country, after three days of fighting between rival factions within the presidential guard.
Fighting started on Sunday after some members of the guard, known as the Tigers, turned against President Salva Kiir, who described the violence as an attempted coup.
He blamed former Vice President Riek Machar, whom he sacked in July after a power struggle.
According to Jok Madut Jok, of the Sudd Institute, a policy and research institution in Juba, mortars and artillery were used as well as small arms:
“I think we are on the verge of civil war,” he said.
Although most of the casualties are soldiers, civilians have also been caught in the crossfire.
The United Nations said on its twitter feed that more than 1,000 civilians had sought refuge in the UN compound in Juba.
According to the UN, late on Tuesday fighting spread to the area around Bor, capital of Jonglei state, 120 miles north of Juba.
“Last night there was fighting in two military barracks,” Hussein Maar, deputy governor of Jonglei state, told Reuters, although he said the town of Bor was calm.
The country has seen fighting between the Dinka and Nuer ethnic groups before – President Kiir is a Dinka, and his former deputy Machar is a Nuer.
But analysts say divisions in South Sudan go deeper, as rival factions running broadly along ethnic lines have emerged in the army, probably beyond the leaders’ control.
Machar is reported to be heading to Bor to join up with his fellow Nuer, Peter Gadet, who has launched a rebellion against the government. Both men command several thousand armed men.
A broader conflict would mean disaster for South Sudan’s civilan population.
Two million were killed in decades of secessionist conflict with the north. South Sudan split from the north of Sudan to form an independent state in August 2011.
South Sudanese hoped that independence would bring peace, security and development, but the government has been mired in corruption and strife.
Civil war would threaten vital aid and could be exploited by the northern government in Khartoum, which has had persistent rows with Juba over undefined borders, oil and security.
The United States, which has urged its citizens to leave South Sudan, said it would start evacuating its people from Juba airport on flights organised by the US State Department.
Britain said it was temporary withdrawing of some embassy staff and dependants.
A Foreign Office (FCO) spokesperson said: “Due to the current instability in Juba the FCO has taken the decision to temporarily withdraw some staff and dependants.
“The Embassy remains open, however should British Nationals in Juba require consular assistance we ask that you contact the FCO in London via text, online or by phone.”
Diplomats said the United Nations had reports of between 400 and 500 people killed and up to 800 wounded.
“Two hospitals have recorded between 400 and 500 dead and (up to) 800 wounded,” a diplomat said on condition of anonymity, citing an estimate United Nations peacekeeping chief Herve Ladsous gave during a closed-door briefing for the 15-member body.
Mr Madut Jok said he saw the bodies of 200 soldiers in the barracks where the fighting started, and another 40 in Juba hospital. “They will be buried in mass graves, because their relatives don’t dare come out and claim them,” he said.
“Most people are scared they might be confronted with a mob or see dead bodies,” said an aid worker in Juba, adding that the city was calmer on Wednesday morning, after residents woke to heavy gunfire and artillery blasts on Monday and Tuesday.
Majok Guangdong Thiep, South Sudan’s ambassador to Kenya, told Kenyan television the airport would re-open on Wednesday. He said the situation in Juba was under control.
However, an official at Kenya airline Fly540 said a flight due to depart at 8am from Nairobi for Juba did not leave because the South Sudanese airport was not open.
“Planes due to take off for Juba are empty as people don’t want to go there. Even the Sudanese don’t want to go home,” said the Fly540 official, who asked not to be identified.