* South Sudan says militia advancing on Bor
* Violence has contributed to higher oil prices
* Neighbours have set Dec. 31 deadline for talks
By Aaron Maasho and Carl Odera
JUBA, Dec 30 (Reuters) - Hundreds of people fled South Sudan's flashpoint town of Bor as the army warned of an imminent attack by the "White Army" militia on Monday, two weeks into ethnic fighting that has ravaged the...