(In MAY 17 story, corrects to remove reference to North Sea production rise forecast, paragraph 10)
* Prices fall two days after Saudi, Russia push for extended cut
* Surprise rise in U.S. API crude stocks weighs on prices
* Rising North Sea, U.S. production undermines OPEC-led cuts
By Henning Gloystein
SINGAPORE, May 18 (Reuters) - Oil prices fell 1 percent on Wednesday after data showed an increase in U.S. crude inventories, stoking concerns that markets remain oversupplied despite efforts by top producers Saudi Arabia and Russia to extend output cuts.
Brent crude futures were down 50 cents, or 1 percent, from their last close at $51.15 per barrel at 0146 GMT.
U.S. West Texas Intermediate (WTI) crude futures were at $48.15, down 51 cents, or 1 percent.
U.S. crude oil inventories rose by 882,000 barrels in the week ending May 12 to 523.4 million, compared with analyst expectations for a decrease of 2.4 million barrels, data from industry group the American Petroleum Institute (API) showed on Tuesday.
The fall in prices came just days after Saudi Arabia and Russia said on Monday that they agreed the need for a 1.8 million barrels per day (bpd) crude supply cut by the Organization of the Petroleum Exporting Countries (OPEC) and some other producers including Russia to be extended for nine months, until the end of March 2018.
"The vulnerability of OPEC's ... rhetoric was starkly revealed ... with both Brent and WTI falling ... as the U.S. API crude inventories showed an unexpected increase," said Jeffrey Halley, senior market analyst at futures brokerage OANDA in Singapore.
The extension of the supply cuts, which started in January and were supposed to end in June, is seen as necessary by some as they have not so far significantly tightened the market or propped up prices.
The International Energy Agency (IEA) said on Tuesday that commercial oil inventories in industrialised countries rose by 24.1 million barrels in the first quarter of the year, a time during which the OPEC-led production cut was already in place.
"The agreement by OPEC to extend cuts into 2018 is critical," said AB Bernstein in a note.
U.S. production C-OUT-T-EIA has jumped by more than 10 percent since mid-2016 to 9.3 million bpd, not far off top producers Russia and Saudi Arabia.
Despite this, Bernstein said the OPEC-led cuts would "lead to accelerated inventory drawdowns in 2H17," although it added that a "return to normalized inventories will ... drag into 2018." (Reporting by Henning Gloystein; Editing by Joseph Radford and Richard Pullin)