* Move threatens to widen fighting into regional conflict
* Neighbours have set Dec. 31 deadline for ceasefire
* South Sudan says "White Army" militia advancing on Bor
* Violence has helped push up oil prices
By Aaron Maasho and Carl Odera
JUBA, Dec 30 (Reuters) - Uganda's president said on Monday East African nations had agreed to unite to defeat South Sudanese rebel leader Riek...