In a broadcast on state television, police chief Abraham Peter Manyuat said the curfew would continue until further notice from 6pm to 6am daily to try to restore security and prevent the destruction of property.
"The police will not tolerate any violations," he said.
Four people were wounded on Thursday night in South Sudan's capital Juba, some by bullets and machetes, when youths in several suburbs looted and vandalised shops of Sudanese people.
On Friday, shops in many Juba suburbs were closed as police and other security forces tried to relocate Sudanese people to safer areas due to fears they could be attacked by rioters.
The riots follow the alleged killing of South Sudanese people by members of Sudan's military and allied groups in the city of Wad Madani in Sudan's El Gezira region.
South Sudan President Salva Kiir Mayardit has warned people not to "turn against Sudanese traders". (AP PHOTO)
On Tuesday the Sudanese army condemned what it called "individual violations" in El Gezira after human rights groups blamed it and its allies for ethnically-targeted attacks against civilians accused of supporting the rebel Rapid Support Forces (RSF).
Sudan's military has been battling RSF fighters in a near two-year civil war.
South Sudan's foreign ministry summoned Sudan's ambassador over the alleged killings earlier this week, and President Salva Kiir Mayardit called for calm.
"It is crucial that we do not allow anger to cloud our judgment or turn against Sudanese traders and refugees currently residing in our country," his office said in a statement.
