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UPDATE 9-Oil prices dive 3 pct to 3-week low; OPEC crude output up

* OPEC oil output rose in May, 1st monthly increase this year

Libya production at 827,000 bpd - NOC

* U.S. crude oil inventories to show 2.5 mln barrel fall - poll

* Russia's May oil output in line with OPEC deal -sources

* Coming Up: API U.S. inventory report at 4:30 p.m. (2030 GMT) (New throughout, updates prices, market activity and comments)

By Scott DiSavino

NEW YORK, May 31 (Reuters) - Oil prices sank 3 percent to a three-week low on Wednesday as an increase in Libyan output helped boost monthly OPEC crude production for the first time this year.

Brent notched its fifth straight monthly decline in a row despite OPEC-led output cuts and forecasts that U.S. crude inventories would fall for an eighth straight week since hitting a record at the end of March.

U.S. crude stocks were forecast to have fallen 2.5 million barrels last week.

The American Petroleum Institute (API) was scheduled to release its data at 4:30 p.m. EDT (2030 GMT) on Wednesday, and the U.S. Energy Information Administration (EIA) report was due at 11:00 a.m. EDT (1500 GMT) on Thursday, both delayed a day because of the Memorial Day holiday on Monday.

Brent crude futures for July fell $1.53, or 3.0 percent, to settle at $50.31 a barrel on their last day as the front-month. It was Brent's lowest close since May 10.

U.S. West Texas Intermediate crude fell $1.34, or 2.7 percent, to settle at $48.32 per barrel, its lowest close since May 12.

Brent's premium over the same U.S. month <WTCLc1-LCOc1> narrowed to its lowest in almost five weeks.

For the month of May, Brent fell almost 3 percent, its fifth straight monthly loss. WTI had its third straight monthly decline, ending May down more than 2 percent.

Output from the Organization of the Petroleum Exporting Countries (OPEC) rose in May, the first monthly increase this year, a Reuters survey found. Higher supply from Nigeria and Libya, OPEC members exempt from a production-cutting deal, offset improved compliance by others.

"Even if Libyan output levels from here for a few weeks, current relative strength provides an additional challenge to OPEC given the fact that the elevated Libyan production is not only eating into other OPEC members market share but is also forcing renewed weakening in Brent structure," Jim Ritterbusch, president of Chicago-based energy advisory firm Ritterbusch & Associates, said in a note.

Libya's oil production has risen to 827,000 bpd, above a three-year peak of 800,000 bpd reached earlier in May, the National Oil Corporation said.

OPEC and other producers, including Russia, agreed last week to extend a deal to cut production about 1.8 million barrels per day (bpd) until the end of March 2018.

Compliance with output cuts remained high among OPEC members and industry sources said Russian figures for May showed output in line with its pledge.

Saudi Arabia and Russia said OPEC and non-OPEC producers were committed to bringing global oil inventories down to the industry's five-year average.

(Additional reporting by Karolin Schaps in London and Henning Gloystein in Singapore; Editing by Marguerita Choy and Keith Weir)