James Wani Igga

South Sudan's current Vice President James Wani Igga, center left, arrives to visit wounded patients at the Juba Military Hospital in Juba, South Sudan, on Saturday. (Ben Curtis / Associated Press / December 28, 2013)

JOHANNESBURG, South Africa — Thousands of armed youths loyal to South Sudan’s rebel leader are preparing to launch an assault on the key town of Bor, a government official said Saturday, as continued fighting in the fragile new country dimmed prospects of a cease-fire.

The South Sudanese government has said it is willing to end hostilities with rebels led by former Vice President Riek Machar, who is in a power struggle with President Salva Kiir.

But Machar has responded warily to the calls by East African countries for a truce, telling the BBC that any talks of a cease-fire must be serious and credible, and negotiated by representatives from both sides.

“Until mechanisms for monitoring are established, when one says there is a unilateral cease-fire, there is no way the other person would be confident this is a commitment,” Machar told the broadcaster.

At least 121,600 people have fled their homes, and thousands are feared dead since fighting broke out Dec. 15, according to the United Nations. Some 63,000 civilians have sought shelter inside U.N. bases.

Government troops are in control of Bor, the strategically important capital of Jonglei state, after it briefly fell to the rebels.

Military spokesman Col. Philip Aguer told reporters Saturday that some 25,000 young Luo-Nuer men, from the same tribe as Machar, were on the march toward Bor.

This youth militia, known as the “White Army” — a name derived from the light-colored ash worn on their skin as an insect repellent — are preparing for an offensive to retake the town, Aguer said.

Bor is where three U.S. military aircraft were about to land when they were hit by gunfire Dec. 21, injuring four American service members.

While the conflict in South Sudan began as a political battle between Kiir and Machar, it has taken on an added tribal element with clashes between the Dinka and Nuer people.

Doctors Without Borders has described the situation in South Sudan as “volatile and rapidly evolving,” limiting the movements of aid workers.

A regional bloc of East African nations on Friday said that negotiations between South Sudan’s government and rebels must begin by Tuesday.

James Wani Igga, the South Sudanese vice president, on Saturday said his government had “positively responded” to the demands of regional heads of state and the international community, but that Machar had put up obstacles to an agreement.

“This is complete intransigence and obstinacy because the main issue now is to stop violence,” Igga is quoted as saying on the South Sudanese government’s official Twitter feed.

Igga is also quoted as saying that “rogue soldiers that return will be treated humanely as brothers and citizens of South Sudan.”

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