South Sudan's President Salva Kiir wipes his face during a news conference in Juba, Dec. 18, 2013. Goran Tomasevic/Reuters

The United Nations said it has discovered a mass grave containing about 75 bodies in South Sudan as the conflict deepened and U.S. officials announced that 150 Marines were deployed to the Horn of Africa to prepare for possible further evacuations of U.S. citizens. The news follows a week of escalating violence after what President Salva Kiir, an ethnic Dinka, called an attempted coup by ousted former Vice President Riek Machar, an ethnic Nuer. To date, more than 1,000 people have been killed, according to the U.N., though no firm numbers are available. U.N. human-rights chief Navi Pillay said in a statement on Tuesday that the mass grave was found in Bentiu, the capital of oil-rich Unity state, and that at least two other mass graves were reportedly found in Juba, the country’s capital. A spokeswoman for the Geneva-based human-rights office says the bodies in Bentiu reportedly were Dinka. Ravina Shamdasani said that the U.N. is investigating the graves and that it is unclear who was responsible for the killings. But representatives of the U.N. Mission in South Sudan disputed the numbers issued in Geneva. The report was an inflation of a "skirmish" that killed 15 people, UNMISS said. The African mission said it is still deeply concerned about extrajudicial killings and is investigating the reports. Ethiopia Foreign Minister Tedros Adhanom told The Associated Press that the leaders of six East African countries will travel to South Sudan Thursday to try to advance peace talks and end 10 days of violence.

Tens of thousands displaced

The United Nations Security Council agreed on Tuesday to almost double the size of the peacekeeping force in troubled South Sudan, adding nearly 6,000 extra soldiers and police. UN chief Ban Ki-moon had called for the UNMISS force to be increased to counter a major outbreak of violence, and member states agreed to increase the military contingent to 12,500 troops. A parallel civilian police deployment will reach 1,323. Ban had earlier asked for a larger increase in the size of th force citing human-rights abuses and “killings fueled by ethnic tensions.” But the vote only increased the maximum permitted size of the force. Member states must still commit more troops to UN command, and Ban warned this "will not happen overnight." In the meantime, Council members demanded an end to hostilities between forces loyal to South Sudan's President Salva Kiir and to his rival deposed vice president Riek Machar. Their statement expressed "grave alarm and concern regarding the rapidly deteriorating security and humanitarian crisis" and warned that those responsible for war crimes would be held responsible. On Monday, after a thwarted evacuation attempt over the weekend in which four U.S. troops were wounded by gunfire, the U.S. military sent to the Horn of Africa a special crisis-response team of Marines normally stationed at Moron air base in Spain. Three U.S. officials, speaking on condition of anonymity, said the Marines were sent to a base in Djibouti, where they could quickly deploy to South Sudan if needed. So far, about 380 U.S. officials and private citizens have been evacuated, according to the State Department. British, Canadian and Kenyan citizens are among 3,000 foreigners trapped in Bor — a flash point of the conflict — experiencing regular bouts of heavy machine-gun fire. Australians, Ugandans and Ethiopians are among 17,000 people seeking protection at a U.N. base in Bor. According to the U.N., tens of thousands have been displaced by the violence and about 45,000 are seeking protection at U.N. bases. Toby Lanzer, the U.N.’s humanitarian coordinator, said there are probably more than 100,000 internal refugees.

War plans