* Move threatens to widen fighting into regional conflict
* No confirmation of pact vs Machar from regional powers
* South Sudan says "White Army" militia advancing on Bor (Adds U.S. comment from paragraph 15)
By Aaron Maasho and Carl Odera
JUBA, Dec 30 (Reuters) - Uganda's president said on Monday the nations of East Africa had agreed to move in to defeat South Sudanese rebel leader Riek Machar if he rejected a ceasefire offer, threatening to turn an outburst of ethnic fighting into a regional conflict.
Hours after President Yoweri Museveni's ultimatum, rebels and the feared "White Army" militia clashed against government troops just outside Bor, the capital of Jonglei state, officials said.
They said the government side was braced for a "full scale" attack on the town, seized by rebels for several days earlier this month and the site of an ethnic massacre in 1991. Thousands of civilians had fled for the surrounding swamps.
Two weeks of clashes have already killed at least 1,000 people in the world's newest nation, unnerved oil markets and raised fears of a civil war in a region ravaged by fighting in Central African Republic and Democratic Republic of Congo.
"We gave Riek Machar four days to respond (to the ceasefire offer) and if he doesn't we shall have to go for him, all of us," Museveni told reporters in South Sudan's capital, Juba, referring to a Dec. 31 deadline.
Asked what that meant, Museveni said: "to defeat him".
He did not spell out whether South Sudan's neighbours had actually agreed to send troops to join the conflict that erupted in Juba on Dec. 15.
But his words underlined the scale of regional concern over the fighting that has spread to South Sudan's oil-producing states - often along ethnic lines, between Machar's group, the Nuer, and President Salva Kiir's Dinka.
Past conflicts in South Sudan have sent refugees pouring over its borders and spurred on rebels in neighbouring countries, including the Lord's Resistance Army in Uganda.
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 Dec. 31 to lay down their weapons.
Kenya's presidential spokesman, Manoah Esipisu, said it would be inappropriate to comment until the deadline has passed. Machar himself did not respond to calls.
Information Minister Michael Makuei said the rebels want to take Bor ahead of the deadline so Machar "can talk from a position of strength" once peace talks start.
"This is why he has been intransigent," Makuei said.
FIGHTING DISPLACES 180,000
The United Nations, Washington, and other Western countries that have poured hundreds of millions of dollars of aid into South Sudan since it won its independence from Sudan in 2011 have also scrambled to stem the unrest.
U.S. Secretary of State John Kerry has had regular contact with regional leaders and spoken almost daily to both Kiir and Machar since Dec. 20 to push for a ceasefire, State Department spokeswoman Marie Harf said.
"What we have said is there is no place for violence here and the sides need to take a step back and move towards a mediated negotiated political dialogue," Harf said.
Meanwhile, U.S. special envoy to South Sudan, Donald Booth, was in Juba on Monday "attempting to work with both President Kiir and former Vice-President Machar to finalise details of a political dialogue and arrange for negotiations to begin in coming days," Harf added.
"It is a very complicated and tenuous situation," she said, adding that 400 U.S. official and private citizens had been evacuated from South Sudan since the trouble erupted. "
Fighting has displaced at least 180,000 people, including 75,000 seeking refuge inside U.N. bases across the country, according to U.N. figures.
Falling global oil prices have been kept in check by fears there could be further cuts to output in South Sudan, which BP says holds the third-largest oil reserves in sub-Saharan Africa after Angola and Nigeria.
South Sudan's oil production has fallen by nearly a fifth to 200,000 barrels per day after oilfields in Unity state were shut last week due to the fighting.
Control Risks analyst Paul Gabriel said Museveni's words were probably aimed at pressing Machar to join talks, rather than a threat of imminent intervention.
There was currently little regional appetite to get involved in the fighting, though "that might change quickly if there is a situation where Juba or President Kiir is threatened," Gabriel added.
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. He has responded coolly to the ceasefire offer and the army, the SPLA (Sudan People's Liberation Army) has said it has continued to fight his soldiers.
"WHITE ARMY" THREAT
"The SPLA forces in Bor town are on maximum alert," SPLA spokesman Philip Aguer said on Monday night after skirmishes with the Nuer "White Army" militia just outside the town.
The White Army - made up of Nuer youths who dust their bodies in white ash - has in the past sided with Machar.
But a spokesman for the government of South Sudan's Unity state, now controlled by forces loyal to Machar, on Sunday denied Machar was in control of the White Army fighters, raising the prospect that the violence was spreading beyond the control of widely recognised ethnic leaders.
Thousands of civilians have fled from Bor over the past few days, crossing the White Nile river and heading for the swamps, Makuei told Reuters. Nuer militias massacred Dinkas in Bor during an outburst of ethnic fighting in 1991.
"They (the White Army) have attacked the village of Mathiang (18 miles (30 km) from Bor), killing civilians and burning civilian houses down. They are butchering civilians," said Bor's mayor, Nhial Majak Nhial, from the town, 190 km (120 miles) north of Juba.
Nhial said he was urging civilians to escape Bor as the White Army militia neared.
The reports of clashes and advances came from remote areas largely inaccessible to journalists and it was not possible to verify them independently.
Tribal elders over the weekend persuaded many of the Nuer youths advancing on Bor to abandon their march, but officials said about 5,000 refused to turn back. (Additional reporting by Lesley Wroughton in Washington, reporting, writing by Drazen Jorgic; Editing by Richard Lough, Andrew Heavens, Philippa Fletcher and Dan Grebler)