Satellite images analyzed by Amnesty show that more than 230 structures were destroyed by fire.
The human rights organization, which says it interviewed dozens of witnesses and victims, reveals that this warning did not prevent the assault.
According to witnesses, attackers shot at people in their homes or as they fled.
They also set fire to homes and the market, looted property and ransacked part of the hospital.
In the 48 hours before the attack, about a thousand people had sought refuge at a temporary UNMISS base located near Abiemnom.
According to Amnesty, the UN organization acknowledged that tensions had been high since mid-February.
Amnesty is calling on the government and UNMISS to conduct independent investigations into this attack.
It is also calling for those responsible to be held accountable.
Independent from Sudan since 2011, South Sudan is plagued by insecurity.