The 15 Council members "expressed horror and anger" over the killings of hundreds of people in the oil town of Bentiu, and "expressed their readiness to consider appropriate measures against those responsible" -- an allusion to possible sanctions.
The Council called for "an immediate end to all human rights violations and abuses and violations of international humanitarian law" and demanded that the government of President Salva Kiir protect civilians.
The United Nations Mission in South Sudan (UNMISS) had said on Monday that both South Sudanese and Sudanese, some from Sudan's Darfur region were killed in "targeted killings" after rebels took Bentiu, the capital of South Sudan's oil-rich Unity state, from government forces last week.
The Security Council called on the office of UN human rights chief Navi Pillay to launch a probe into what happened in Bentiu.
Council members "deplored strongly the systematic targeting of civilians based on their ethnicity; the pursuit and execution of civilians inside places of safety, including a mosque, a church, and a hospital; and the use of radio broadcasts to foment hate and sexual violence.