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 that has threatened 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. Mr. 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 December 15, pitting soldiers from Mr. 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.

Mr. Leuth, the Information Minister, said the government has not yet established formal contact with Mr. 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 Mr. 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 Mr. Machar. There was also reported fighting overnight in Malakal, the capital of Upper Nile state, according to Mr. 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 per cent 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 in South Sudan.

“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.

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