The United Nations will nearly double its peacekeeping force in South Sudan, attempting to quell ethnic and political violence that has swept over the world's youngest country in the past week.

Thousands have been killed there, according to the top U.N. humanitarian officer in the country, Toby Lanzer, as President Salva Kiir's government forces have clashed with forces allied with former Vice President Riek Machar. At least three mass graves have reportedly been discovered, U.N. human rights chief Navi Pillay said.

The U.N. peacekeeping mission there will increase from about 7,000 to nearly 14,000 soldiers and police, after a U.N. Security Council vote today. The U.N. forces will be transferred from U.N. missions in the Democratic Republic of the Congo, Cote d'Ivoire, Liberia and nearby Darfur and Abyei, the United Nations announced.

The added forces will primarily protect civilians, about 45,000 of whom have taken shelter in U.N. bases, according to the U.N. special representative to South Sudan, Hilde Johnson.

"Attacks on civilians and the U.N. peacekeepers must cease immediately," U.N. President Ban Ki-Moon said. "The United Nations will investigate reports of these incidents and of grave human rights violations and crimes against humanity. Those responsible will be held personally accountable. They should know the world is watching."

President Kiir announced Dec. 16 that a coup by Machar's forces had been prevented, but it was unclear whether one had actually been attempted. Since then, political violence has become ethnic, according to U.N. reports.

Kiir is an ethnic Dinka, while Machar is an ethnic Nuer.