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 US and Nigeria were expected in the capital, Juba, following a mission by foreign ministers from East Africa and the Horn.
Mr Obama revealed that US troops attacked by unidentified gunmen on 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 was in addition to another 45 troops sent this week to help protect US citizens, personnel and property at Washington’s embassy in 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,” Mr Obama wrote in a letter to congress.
Foreign governments, including in Britain, Kenya, Lebanon, Uganda and the US, have been evacuating their citizens. The US earlier safely evacuated its citizens from Bor, a day after an aborted mission in which four US servicemen were wounded.
Mr Obama has called for an end to the violence, warning that 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.
Fighting has gripped South Sudan for a week, after President Salva Kiir accused former deputy Riek Machar of attempting a coup. Mr Machar denies the claim and says Mr Kiir carried 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 Mr Kiir, an ethnic Dinka, battle forces backing Mr Machar, a Nuer.
UN Secretary-General 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. The UN said 20 Dinka 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 Mr Machar are in control of Bor – the capital of Jonglei state, situated about 200km north of Juba – although South Sudan’s army spokesman said government troops were advancing to retake the town.
Pariang is in Unity state, the country’s main oil-producing area, which is in rebel hands.
The UN said it had begun evacuating “non-critical” staff from Juba, where nearly 20,000 civilians had taken refuge at UN bases. Its humanitarian chief in South Sudan warned that the country had “unravelled.”
Toby Lanzer, also the deputy head of UNMISS, said the crisis was the result of “an armed struggle within the ruling party with innocents stuck in the middle”.
An official in Bentiu, the rebel-held capital of Unity state, said the area was littered with bodies after the town’s fall, which was speeded by the defection of a top government commander. “There are so many bodies, over 100 not yet buried,” the official said.