The United Nations estimates at least 400 people have been killed in several days of fighting in South Sudan. Two members of a peacekeeping unit were killed this week in an attack on their compound in restive Jonglei state.
Netsanet Belay, director of African programs at Amnesty International, said Friday there was growing evidence troops and armed civilians from the Dinka and Nuer ethnic communities were targeting civilians.
"The fact that these attacks were carried out by armed youths is a disturbing sign that this conflict is moving beyond fighting between soldiers and into widespread inter-communal violence," he said in a statement.
Conflict erupted after South Sudanese President Salva Kiir accused former Vice President Riek Machar, sidelined in a summer Cabinet reshuffling, of plotting a coup.
Western embassies have called on their nationals to evacuate as a security precaution.
Emma Vickers, a campaigner at advocacy group Global Witness, called on warring factions to sit down at the negotiating table and settle their differences peacefully.
"This week's fighting is a sober reminder of the fragile peace in South Sudan, where civilians are bearing the brunt of political infighting," she said in a statement.