JUBA- President Barack Obama Sunday firmly vowed to take more action in South Sudan if needed amid growing fighting after deploying extra US troops, as the United Nations promised to send more peacekeepers.
The announcements came as world leaders embarked on a diplomatic push to pull the world's youngest nation back from the brink of all-out civil war.
Special envoys from the United States and Nigeria were expected in the capital Juba following a mission by foreign ministers from east Africa and the Horn.
Obama revealed that US troops attacked by unidentified gunmen Saturday as they approached the rebel-held city of Bor aboard CV-22 Osprey tiltrotor aircraft were part of a unit of about 46 troops sent that day to help evacuate Americans.
That contingent was in addition to another 45 troops sent this week to help protect US citizens, personnel and property at Washington's embassy in the capital Juba.
"As I monitor the situation in South Sudan, I may take further action to support the security of US citizens, personnel and property, including our embassy, in South Sudan," Obama wrote in a letter to Congress.
Foreign governments, including in Britain, Kenya, Lebanon, Uganda and the US, have been evacuating their nationals.
The United States earlier safely evacuated US nationals from Bor, a day after the aborted mission in which four US servicemen were wounded.
Obama has called for an end to the violence, warning the country was on the "precipice" of civil war and that any military coup would trigger an end to diplomatic and economic support from Washington and its allies.
More fighting triggers growing outrage
Fighting has gripped South Sudan for a week, after President Salva Kiir accused his former deputy Riek Machar of attempting a coup. Machar has denied the claim, and has accused Kiir of carrying out a vicious purge of his rivals.
The clashes have left hundreds dead and sent tens of thousands of people fleeing for protection in UN bases or to safer parts of the country, which only won independence from Sudan in 2011, but remains blighted by ethnic divisions, corruption and poverty.
There are both ethnic and political dimensions to the fighting, as troops loyal to Kiir, an ethnic Dinka, battle forces backing Machar, a Nuer.
UN chief Ban Ki-moon called for an immediate end to the violence three days after two Indian peacekeepers were murdered when a UN compound where civilians were sheltering was attacked in the powder-keg state of Jonglei.
According to the UN, some 20 Dinka ethnic civilians were also killed in the assault.
The UN mission in South Sudan (UNMISS) announced plans to reinforce its military presence in Bor and Pariang to help protect civilians.
Forces loyal to Machar are currently in control of Bor, the capital of Jonglei state situated about 200 kilometres (125 miles) north of Juba, although South Sudan's army spokesman said government troops were advancing to retake the town.
Pariang is located in Unity state, the country's main oil-producing area, which the government has acknowledged is currently in the hands of the rebels.
The UN said it had begun evacuating "non-critical" staff from Juba, where nearly 20,000 civilians have taken refuge at UN bases.
The world body's humanitarian chief in South Sudan warned that the country has "unravelled."
Toby Lanzer, who also serves as deputy head of UNMISS, said the crisis was the result of "an armed struggle within the ruling party with innocents stuck in the middle."
Bodies in the streets
A local official in Bentiu - the rebel-held capital of Unity state - said the area was littered with bodies following the fall of the town, which was speeded by the defection of a top government commander.
"There are so many bodies, over a hundred not yet buried," the local official, who asked not to be named, told AFP.
A witness from the area also said unidentified militiamen have been roaming the area for days, setting up road blocks and robbing passers-by.
The witness, who asked not to be identified, suggested fighters from nearby Darfur in Sudan may also have moved to the area to profit from the instability, adding there were unconfirmed reports of rapes.
Sudan People's Liberation Army spokesman Philip Aguer confirmed that "Unity state is currently divided, with the SPLA and the loyalists to the government on one side and those who are supporting Riek Machar on the other."
Oil production accounts for more than 95 per cent of South Sudan's fledgling economy, and the sector has been hit with oil companies also flying out their employees after the death of at least five South Sudanese oil workers Wednesday.
Juba's ambassador to Khartoum, however, insisted that oil facilities had not been damaged and oil was still flowing.