* Brent, WTI crude both near mid-2015 highs
* OPEC, Russia-led supply cuts prop up prices
* Libya, North Sea pipeline outages also support crude
* But both pipelines to return to normal operation in Jan
* Soaring U.S. oil output undermines efforts to cut supplies (Adds API data, updates prices)
SINGAPORE, Dec 28 (Reuters) - Oil prices were stable on Thursday with trading activity drying up ahead of the New Year weekend.
Heading into 2018, traders said market conditions were relatively tight due to ongoing supply cuts led by the Middle East-dominated Organization of the Petroleum Exporting Countries (OPEC), as well as top producer Russia.
U.S. West Texas Intermediate (WTI) crude futures were at $59.69 a barrel at 0336 GMT, up 5 cents from their last settlement. WTI broke through $60 a barrel earlier this week, the first time since June 2015.
WTI received support from a report by the American Petroleum Institute (API) showing a 6 million barrel drop in crude oil inventories to 432.8 million.
Brent crude futures were at $66.50 a barrel, up 6 cents. Brent broke through $67 earlier this week, the first time since May 2015 this week.
Traders said the high prices were a result of a relatively tight market following a year of OPEC and Russia-led production cuts, which were started last January and scheduled to cover all of 2018.
Pipeline outages in Libya and the North Sea have also been supporting oil prices.
"Given the much stronger price response to supply disruptions in the wake of OPEC supply cuts, the market is poised to make further gains," said Stephen Innes, head of trading for Asia/Pacific at futures brokerage Oanda in Singapore.
"With geopolitical risk no less sure ahead of Libyan elections next year, we should expect more regional chaos and disorder to underpin oil prices," he added.
Around 100,000 barrels per day (bpd) in oil supplies were disrupted in Libya this week after an attack on a pipeline.
In the North Sea, the 450,000 bpd capacity Forties pipeline system was shut earlier this month due to a crack.
Both pipelines are expected to return to normal operations in January, with Forties already in the start-up process.
A major factor countering efforts by OPEC and Russia efforts to prop up prices is U.S. oil production <C-OUT-T-EIA>, which has soared more than 16 percent since mid-2016 and is fast approaching 10 million bpd.
Only OPEC king-pin Saudi Arabia and Russia produce more.
The latest official U.S. production figures are due to be published by the on Thursday.
(Reporting by Henning Gloystein; Editing by Michael Perry and Kenneth Maxwell)