The U.N. Security Council is expressing rare "horror" at the recent massacre of civilians in South Sudan and threatens sanctions along with an urgent investigation in Bentiu, according to Reuters.

The strongly worded press statement released Thursday deplores the killings last week in Bentiu, the capital of oil-producing Unity state, as well as the use of radio broadcasts to "foment hate and sexual violence," Reuters reported.

The council statement also expresses "grave concern" over the more than 23,000 displaced people who are seeking shelter at the U.N. camp in Bentiu as the risk of a humanitarian crisis grows, according to Reuters.

It also says council members "indicated their willingness to take additional measures," meaning sanctions, if attacks on civilians continue amid growing ethnic tensions, Reuters reported.

The council statement calls on the government of the world's youngest nation to ensure the safety of civilians and denounce attacks, according to Reuters.

More than 1 million people have fled their homes and thousands of people have been killed since fighting erupted in December between troops backing President Salva Kiir and soldiers loyal to his sacked deputy, Riek Machar, Reuters reported.

The fighting has exacerbated ethnic tensions between Kiir's Dinka people and Machar's Nuer. Negotiations between the Kiir government and rebels loyal to Machar have failed to advance since the Jan. 23 signing of a ceasefire that never took hold, according to Reuters.

South Sudan's government said on Thursday it would free four high-profile political prisoners facing treason charges, meeting a demand by rebels in the faltering peace process, Reuters reported.

The council demanded that Kiir, Machar and other leaders "demonstrate leadership by publicly stating that any and all attacks on civilians are unacceptable, committing to hold accountable those who order such attacks or carry them out, immediately ending the violence, and returning to the inclusive peace process," according to Reuters.