Four US service personnel have been wounded after an aircraft they were on was shot at on an evacuation mission in South Sudan, the US military says.
The aircraft was attacked as it approached Bor, which is occupied by forces loyal to the former Vice-President Riek Machar, it added.
South Sudan has been in turmoil since President Salva Kiir accused Mr. Machar a week ago of attempting a coup.
Machar told the BBC the rebels were under his control.
He was in control of large parts of the country, he said, and troops loyal to him had also seized control of Unity, a state on the border with Sudan which produces much of the country’s oil.
He added that he was prepared to negotiate with the government if recent political detainees were released.
At least 500 people have been killed since the fighting began.
Ugandan Involvement …
The US military statement said the aircraft, which it did not identify, was involved in the evacuation of US citizens from Bor.
“After receiving fire from the ground while approaching the site, the aircraft diverted to an airfield outside the country and aborted the mission,” the statement said.
The aircraft returned to Uganda’s Entebbe airport near the capital Kampala, from where the wounded were taken on to Nairobi, Kenya, Ugandan officials said.
Ugandan media said that two Ugandan aircraft were also involved in the incident.
Uganda is one of a number of other countries trying to evacuate their citizens from South Sudan.
It has sent troops to take part in the operation. They will also try to secure the capital Juba, just 75km (50 miles) from the border, reports say.
However, Uganda denied reports that it has been helping the South Sudanese army by bombing Mr. Machar’s forces in eastern Jonglei state, of which Bor is the capital.
The army is trying to retake Bor. Jonglei is one of the most volatile regions in the country.
Troops backed by helicopter gunships were advancing on the town, army spokesman Philip Aguer told the French AFP news agency.
In Unity state, a major oil-producing region, a senior commander, General James Koang, was reported to have defected to Machar’s forces.
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.
A resident in Unity state told the BBC that Gen Koang announced on local radio he had joined Machar’s rebellion.
On Friday, African mediators held talks with Kiir in an attempt to avert civil war.
The talks are set to continue and US Secretary of State John Kerry said he was sending a special envoy, Ambassador Donald Booth, to help foster dialogue.
President Kiir, a member of the majority Dinka ethnic group, sacked Machar, who is from the Nuer community, in July.
He said that last Sunday night uniformed personnel opened fire at a meeting of the governing party, the Sudan People’s Liberation Movement (SPLM).
Violence then broke out in Juba and has since spread across the country, pitting gangs of Nuer and Dinka against each other.
The whereabouts of Machar, who has denied trying to stage a coup, remain unknown.
Thousands of civilians have flocked to UN compounds seeking shelter from the unrest.
The UN on Friday condemned an attack on its compound in Akobo, Jonglei state, a day earlier in which two Indian peacekeepers and at least 11 civilians were killed.
Jonglei state has seen some of the worst violence since South Sudan became independent from Sudan in 2011, with hundreds killed in periodic clashes between rival heavily-armed ethnic militias sparked by cattle-rustling.
Sudan’s arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
Following decades of conflict, weapons are widely available across much of South Sudan.
South Sudan’s government insists the clashes are over power and politics, not between ethnic groups.
President Kiir said the majority of those arrested after Sunday’s alleged coup attempt were Dinka, not Nuer.
The oil-rich country has struggled to achieve a stable government since becoming independent.
Both Sudan and the South are reliant on oil revenue, which accounts for 98% of South Sudan’s budget. They have fiercely disagreed over how to divide the oil wealth of the former united state – at one time production was shut-down for more than a year. Some 75% of the oil lies in the South but all the pipelines run north.