New York's main contract, West Texas Intermediate for February delivery, was up 13 cents at $99.35 in afternoon trade while Brent North Sea crude for February gained 17 cents to $112.07.

Sanjeev Gupta, head of the Asia-Pacific oil and gas practice at consultancy firm EY, said oil prices were being supported by "fears of disruptions in supply from South Sudan".

But "markets are likely to remain range-bound till the release of new economic data during the first of week of 2014", he said.

Violence in South Sudan, a fledgling oil producer, escalated Wednesday as its army battled rebel forces in a key oil-producing state, while the United Nations moved to double its peacekeeping force to stave off civil war.

Thousands are believed to have been killed in more than a week of violence pitting troops loyal to President Salva Kiir against those backing his rival Riek Machar, a former vice president who was sacked in July.

Oil production, which accounts for more than 95 percent of South Sudan's economy, has been dented by the violence, with oil workers evacuated last week.

The escalating violence has added to concerns about a disruption in global supply, following the continued curtailment of output from OPEC member Libya due to a months-long blockade of crucial terminals in the eastern part of the country.

Investors are also awaiting the weekly US oil inventories report to be released Friday for clues about US demand.

Analysts project US supplies fell 2.2 million barrels, according to a survey by the Wall Street Journal.