Civilians arrive for shelter at the United Nations Mission in the Republic of South Sudan compound in Bor, South Sudan in this Dec. 18, 2013, picture provided by the UNMISS. Handout/Reuters

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.