Updated

The leaders of Kenya and Ethiopia arrived in South Sudan on Thursday to try and mediate between the country's president and the political rivals he accuses of attempting a coup that the government insists sparked violence threatening to destroy the world's newest country.

Kenyan President Uhuru Kenyatta and Ethiopian Prime Minister Hailemariam Desalegn will meet with South Sudanese President Salva Kiir later on Thursday, said Information Minister Michael Makuei Lueth.

World leaders have urged the country's leaders to stop the violence in which thousands are feared killed. The United States, Norway and Ethiopia are leading efforts to open peace talks between Kiir and his political rivals. Kiir said in a Christmas address that he is willing to "dialogue" with all his opponents.

The United Nations is investigating reports of mass killings since violence began spreading across South Sudan after a fight among the presidential guards on Dec. 15, pitting soldiers from Kiir's Dinka ethnic group against those from the Nuer ethnic group of former Vice President Riek Machar. South Sudan's top U.N. humanitarian official, Toby Lanzer, said on Monday that he believes the death toll has surpassed 1,000.

Leuth, the information minister, said the government has not yet established formal contact with Machar, who has been accused of leading what the government insists was a failed coup plot, because "the rebel" was expected to first renounce violence.

"For us, we are not talking with him," he said, referring to Machar, whose whereabouts remain unknown.

Government troops are trying to retake control of Bentiu, the capital of oil-rich Unity state, from forces loyal to Machar. There was also reported fighting overnight in Malakal, the capital of Upper Nile state, according to Lueth. Upper Nile and Unity comprise the country's key oil-producing region, raising fears unrest there could cut off the country's economic lifeblood. South Sudan gets nearly 99 percent of its government budget from oil revenues.

"We are moving toward them and we will flush them out like we did in Bor," he said, referring to the capital of Jonglei state that government troops retook from renegade forces earlier in the week.

Although the capital, Juba, is now calm, fighting appears to spread across the country, stretching the limits of humanitarian workers and aid agencies. The U.N. humanitarian office said aid agencies need $166 million to save lives amid continuing violence.

"The resources will be used to provide clean water and sanitation, health care, shelter, and deliver food and livelihood assistance," the office said in a statement. "It will also ensure that the rights of vulnerable people, including survivors of violence, are better protected. The money will be used to manage sites for displaced people and transport aid workers and supplies to strategic locations where communities are most at risk."

Some 58,000 people have taken refuge in and around U.N. bases in the country and more than 92,000 have fled their homes as a result of fighting that has raised fears of a civil war in the country, according to the United Nations.

South Sudan peacefully broke away from Sudan in 2011 following a 2005 peace deal. Before that, the south fought decades of war with Sudan. The country, one of the world's least developed, still has pockets of rebel resistance and sees cyclical, tribal clashes that result in hundreds of deaths.