"Although equipped for combat, these additional personnel are deployed for the purpose of protecting US citizens and property," Obama wrote in a letter to Congress released by the White House.
Obama also said that 130 more personnel were prepositioned in Djibouti and ready to deploy if necessary.
While US troops will not have a combat role, their presence will not go unnoticed by rival factions.
South Sudan, shepherded into existence by US cash and diplomacy in 2011, has faltered badly in its infancy, and the Obama administration has been accused of abandoning the fragile nation.
Multiple agreements and ceasefires have failed, leaving violence and human rights atrocities unchecked.
At least 272 people have been killed in the latest spasm of violence, although a fragile ceasefire has now entered into force.
At least 42,000 people have fled their homes.
During a visit to Kenya last year, Obama put his weight behind efforts for tough action against South Sudan's warring leaders if they fail to curb the violence.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)