Uganda says region ready to take on, defeat S. Sudan rebel leader By Aaron Maasho and Carl Odera JUBA (Reuters) - Uganda's president said on Monday east African nations 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. 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," Ugandan president Yoweri Museveni told reporters in South Sudan's capital, Juba. Asked what that meant, Museveni said: "to defeat him." He did not spell out whether South Sudan's neighbors 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 - 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 neighboring 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, who have been trying to mediate and last week gave the sides until December 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. The United Nations, Washington, and other Western countries who 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. 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 were on Monday 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, 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. Machar has responded coolly to the ceasefire offer and the army has said it has continued to fight his soldiers. Thousands of people fled South Sudan's flashpoint town of Bor as the army warned of an imminent attack by the Nuer "White Army" militia on Monday, officials said. 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-recognized ethnic leaders. Civilians had fled the town, crossing the White Nile river and heading for the swamps, Information Minister Michael Makuei told Reuters. Nuer militias massacred Dinkas in Bor during an outburst of ethnic fighting in 1991. They have attacked the village of Mathiang (18 miles 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 by road. Nhial said he was urging civilians to escape Bor, the capital of Jonglei state, 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 and writing by Drazen Jorgic; Editing by Richard Lough and Andrew Heavens