* U.N. says up to 500 killed so far
* Regional states sending mediators on Thursday
* President and rival vice president from rival clans (Adds details of oil workers, comments on Bor fighting)
By Carl Odera and Andrew Green
JUBA, Dec 19 (Reuters) - Government forces battled rebels to regain control of a flashpoint South Sudanese town on Thursday, the fifth day of a conflict that has ...