Civilians flee as S Sudan government prepares assault on rebel-held Bor

South Sudanese troops are readying an assault on the rebel-held town of Bor, the president said Monday, after the evacuation of foreign nationals from the capital of a key oil-producing state Jonglei, raising fears of an all-out ethnic civil war in the world’s newest country.

The army is “now ready to move to Bor,” President Salva Kiir told parliament, adding that the counter-attack to wrest back the town after it was captured Wednesday was delayed until U.S. citizens had been airlifted out.

The State Department said Sunday that its citizens were safely evacuated from Bor to Juba, the capital of South Sudan.

The United Nations Mission in South Sudan (UNMISS) also evacuated injured civilians and non-essential staff from its compound in Bor to Juba Sunday, and that they would be evacuated further to neighboring Uganda.

Toby Lanzer, United Nations Deputy Special Representative to the Secretary-General, said that the situation in Bor has grown steadily worse.

“(The) situation up there is very, very tense,” he said. “As I left our base and went to the airport in Bor, there was a lot of looting, a lot of gunshots, a lot of dead bodies.”

Tens of thousands have fled to U.N. compounds or safer areas of the country. Footage released by UNMISS Sunday showed huge crowds outside their compound in Bor – an estimated 15,000 were sheltering inside.

Fighting has gripped South Sudan for more than a week, after Kiir accused his former deputy Riek Machar, who was fired from the government in July, of attempting a coup.

Machar denied the claim and accused Kiir of carrying out a vicious purge of his rivals. Vowing to oust Kiir, his forces have since seized the town of Bor, just 125 miles north of Juba, as well as the town of Bentiu, capital of crucial oil-producing Unity state.

Speaking to Al Jazeera on Sunday, Machar said he wants to be the next leader of the country and intends to run for president in 2015. He said that Kiir should step down.

There are both ethnic and political dimensions to the fighting, as troops loyal to Kiir, an ethnic Dinka, battle forces backing Machar, a Nuer.

Hundreds have been killed in the fighting and world leaders are concerned about civil war in a country with a history of ethnic violence and divided military loyalties.

Sunday, government officials admitted that control of Bentiu, the capital of the oil-rich Unity State, had been seized by a commander who declared support Machar, whom he promptly named governor of the state.

The government insisted it was still in control of the state's oil fields.

South Sudan nets nearly 99 percent of its government budget from oil revenues. The country reportedly earned $1.3 billion in oil sales in just five months this year, according to the London-based watchdog group Global Witness.

U.N. Secretary General Ban Ki-moon told a news conference in Manila on Saturday that the U.N. planned to send resources from other peacekeeping missions in the region to South Sudan.

"We are now actively trying to transfer our assets from other peacekeeping missions like MONUSCO (in the Democratic Republic of Congo) ... and some other areas," he said. "And we are also seeking support from other key countries who can provide the necessary assets."

Ban urged South Sudan's leaders "to do everything in their power" to stop the violence. Foreign ministers from neighboring countries Kenya, Ethiopia, Uganda and Djibouti were in South Sudan earlier in the week to try to diffuse the crisis.

On Saturday, the government's foreign minister told Al Jazeera that South Sudan's president is ready to begin talks "with any rebel group."

South Sudan's Foreign Minister, Barnaba Marial Benjamin, said "our president agreed yes, indeed he is willing for talks with the main group. Why would the government not talk to people who were a part of the struggle?"

South Sudan, which became independent in 2011 after decades of a brutal war with Sudan, has been plagued by ethnic discord, corruption and conflict with Sudan over oil revenues.

Although the south inherited three-quarters of Sudan's oil production when it declared independence, its oil exports are pumped through pipelines running north, raising concern that a rebel takeover of southern oil fields could invite Sudan into the conflict.

Al Jazeera and wire services