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UPDATE 6-Oil prices slip ahead of key OPEC meeting

* OPEC meeting in Vienna on Friday

* OPEC to discuss potential extension of production cuts

* U.S. crude production has reached pre-Harvey storm levels (Updates comment in paragraphs 4-5)

LONDON, Sept 21 (Reuters) - Oil prices slipped on Thursday, giving up some recent gains before a meeting of oil producers that could extend production limits aimed at clearing a glut in supplies that has depressed the market for more than three years.

Ministers from the Organization of the Petroleum Exporting Countries, Russia and other producers meet in Vienna on Friday and are due to consider extending output cuts that began in January.

OPEC and its allies have agreed to reduce output by about 1.8 million barrels per day (bpd) until March 2018 in an attempt to empty inventories. Many analysts now expect them to extend the deal, possibly to the end of next year.

"The bull run in the oil market is running out of steam as unease builds ahead of tomorrows OPEC/non-OPEC meeting," said Stephen Brennock, analyst at London brokerage PVM Oil Associates.

"The (oil futures) contracts have performed well in recent sessions but are struggling for traction," Brennock added.

Brent crude oil was down 40 cents at $55.89 a barrel by 1115 GMT. U.S. light crude was 50 cents lower at $50.19.

Both contracts have risen more than 15 percent over the last three months as global oil supply has tightened.

OPEC's efforts have been hampered by higher production in some other parts of the world, including the United States, where shale oil production is reaching record highs.

Recent hurricanes in the Gulf of Mexico have also pushed up crude oil inventories in some parts of the United States as U.S. refineries have been shut by flooding.

U.S. commercial crude oil stocks <C-STK-T-EIA> rose for a third straight week, building by 4.6 million barrels in the week ending Sept. 15 to 472.83 million barrels.

U.S. oil production has reached 9.51 million bpd, up from 8.78 million bpd directly after Hurricane Harvey hit the U.S. Gulf Coast. <C-OUT-T-EIA>

U.S. crude received some support from a strong draw in gasoline stocks by 2.1 million barrels to 216.19 million barrels, traders said.

The structure of oil futures prices suggests OPEC production cuts are beginning to have an impact, analysts say.

Front-month Brent futures have risen sharply in recent months, much more than forward prices. This has changed the Brent price curve, moving it into what traders call "backwardation," when prices for immediate delivery are higher than prices for later barrels.

The shift is seen as an indicator of a tightening market as it encourages the immediate sale of oil rather than holding it in storage.

(Additional reporting by Henning Gloystein in Singapore; Editing by Greg Mahlich)