At least two US evacuation flights from South Sudan have been fired on, causing some injuries, say reports from Uganda, where the planes landed.

The aircraft were attacked near Bor, which is occupied by forces loyal to former Vice-President Riek Machar, Ugandan officials say.

South Sudan has been in turmoil since President Salva Kiir accused Mr Machar a week ago of attempting a coup.

At least 500 people have been killed in clashes.

Details of the plane incident are still sketchy, but it appears at least one of them was hit, and several US personnel were injured.

The aircraft returned to Uganda's Entebbe airport near the capital Kampala.

A number of other countries are trying to evacuate their citizens from South Sudan.

Uganda 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.

China National Petroleum - one of the main operators - said it was evacuating workers back to Juba.

Commander 'defects'

The South Sudanese army is trying to retake Bor, the capital of eastern Jonglei state, one of the most volatile regions in South Sudan.

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, is reported to have defected to Mr Machar's forces.

Mr 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.

Government troops are patrolling the capital, Juba

A resident in Unity state told the BBC that Gen Koang announced on local radio he had joined Mr Machar's rebellion.

On Friday African mediators held talks with Mr 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 Mr 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 Mr Machar, who has denied trying to stage a coup, remain unknown.

Worst violence

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.