According to Associated Press and Human Rights Watch, forces loyal to the South Sudanese President Salva Kiir attacked a humanitarian compound in the capital Juba in July - raping, robbing and beating dozens of people.
It's part of an ongoing civil war between forces local to the President and those loyal to an opposition group led by former Vice-President Riek Machar. The conflict is ethnic as well as political, with those following Kiir generally associated with the Dinka tribe and those following Machar generally associated with the Nuer tribe.
It's seen millions displaced and thousands die.
Associated Press and Human Rights Watch UN peacekeeping forces, some of which were only a few minutes away, were notified via electronic messages of the attack minutes after it happened. Some aid workers also escaped and notified peacekeeping soldiers at their own bases hours later.
Despite this the sources say UN peacekeeping forces refused to intervene, only rescuing the aid workers after the soldiers had left.
Speaking on Premier's News Hour from Juba, Perry Mansfield from the Christian charity World Vision said: "We had known about this for some time, though certainly this week we learned a lot more information with the press releases that came out.
"South Sudan is the second-most dangerous site for humanitarian work right after Afghanistan and 59 aid workers have been killed since the outbreak of hostilities in 2013.
"We have full time security staff... round our compounds and our housing.
"The situation has just continually deteroriated. Peace is critical - the country is in a desperate situation."
A spokesman for the UN Secretary-General Ban Ki Moon said: "The Secretary-General is alarmed by the preliminary findings of a fact finding investigation by the United Nations Mission in South Sudan (UNMISS) into the attack on Hotel Terrain in Juba on 11 July, in which one person was killed and several civilians were raped and brutally beaten by men in uniform.
"The Secretary-General is also concerned about allegations that UNMISS did not respond appropriately to prevent this and other grave cases of sexual violence committed in Juba.
"Due to the gravity of these incidents, related allegations and the preliminary findings by UNMISS, the Secretary-General has decided to launch an independent special investigation to determine the circumstances surrounding these incidents and to evaluate the Mission's overall response."
Listen to Premier's Hannah Tooley speaking to Perry Mansfield on the News Hour:
