Uganda's president has warned South Sudanese rebel leader Riek Machar against rejecting the government's offer of a ceasefire, saying the nations of East Africa could move in to "defeat" him.
Ugandan President Yoweri Museveni told reporters in South Sudan's capital Juba on Monday that a regional bloc known as IGAD had given Machar "four days to respond" to the ceasefire offer.
"If he doesn't we shall have to go for him, all of us," he said, referring to IGAD.
Hours after President Yoweri Museveni's ultimatum, rebels and militia clashed against government troops just outside Bor, the capital of Jonglei state, officials said.
Bor has become the focus of a bloody power battle between President Salva Kiir and Machar, his former vice president, that has left more than 1,000 dead in the past two weeks. More than 180,000 people have been displaced.
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UN reconnaissance planes have spotted "large" groups of armed youths and regular troops who have defected from Kiir's side north of Bor, UN spokesman Martin Nesirky told reporters.
Bor remains under mainly government control but thousands are fleeing the city, he said.
"Thousands of civilians have been seen heading south on the road to Juba during the course of today, motivated one assumes primarily by fears of an assault on Bor by these groups," he added.
While the root of the conflict is political, it is fast spiralling into an ethnic conflict. Bor is capital of Jonglei state, which has a bitter history of conflict between the ethnic Nuer who mainly follow Machar and the Dinka, Kiir's tribe.
In 1991, an estimated 2,000 people were killed in a massacre in Bor blamed on the Nuer.
Uganda's Museveni did not spell out whether South Sudan's neighbours had actually agreed to send troops to join the conflict that erupted in Juba on December 15.
But his words underlined the scale of regional concern over the fighting that has spread to South Sudan's oil-producing states.
There was no immediate confirmation of the pact to take on Machar from other East African countries, which have been trying to mediate and last week gave the sides until December 31 to lay down their weapons.
Uganda's influence is strong in South Sudan, where special forces from the neighbouring country have been deployed at the request of President Kiir, raising questions about the impartiality of Uganda as a possible mediator.
Kiir sacked his longtime political rival Machar in July and then accused him of starting the December fighting to try to seize power.
Machar denied that charge, but has since retreated into the bush and acknowledged he is leading rebel fighters. Days ago he responded coolly to the ceasefire offer in an interview ith BBC, saying that he had a negotiation team ready but that a ceasfire had to be "serious, credible and properly monitored".
"So until mechanisms for monitoring are established, when one says there is a unilateral ceasefire, there is no way that the other person would be confident that this is a commitment," he said.," he told the television station.
Meanwhile, the army, the SPLA (Sudan People's Liberation Army), has said it has continued to fight Machar's soldiers.