Fighting intensified as the army moved on a key rebel-held town, even as government and rebel negotiating teams gathered at a luxury hotel in neighbouring Ethiopia's capital Addis Ababa.
"We have enough forces who will defeat the rebels within 24 hours," army spokesman Philip Aguer said in South Sudan, with reports of heavy battles involving tanks and artillery on the outskirts of Bor, a dusty town that has already exchanged hands three times since fighting began.
The US embassy in South Sudan ordered a further pullout of staff and urged all citizens to leave on an evacuation flight it had organised because of the "deteriorating security situation."
The ongoing battles prompted the top UN aid official in South Sudan, Toby Lanzer, to warn that soldiers and rebels must protect civilians and aid workers, or risk worsening a situation he described as "critical".
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But in the calm of the hotel in Addis Ababa, rivals met special envoys from regional nations, ahead of direct talks that sources suggested may not take place until at least tomorrow.
Thousands of people are feared to have been killed in the fighting, pitting army units loyal to President Salva Kiir against a loose alliance of ethnic militia forces and mutinous army commanders nominally headed by ex-vice president Riek Machar.
Fighting erupted on December 15 when Kiir accused Machar of attempting a coup in the oil-rich but impoverished nation.
Machar denied this, in turn accusing the president of conducting a violent purge of his opponents and refusing to hold direct talks with Kiir.