* Libyan oil production to reach 800,000 bpd this week
* OPEC-led efforts already undercut by soaring U.S. output (Adds chart, bullet points, updates prices)
SINGAPORE, May 31 (Reuters) - Oil prices fell on Wednesday, as rising output from Libya added to concerns about increasing U.S. production that is undermining OPEC-led production cuts aimed at tightening the market.
Brent crude futures, the international benchmark for oil prices, were at $51.66 per barrel at 0356 GMT, down 18 cents, or 0.4 percent, from their last close.
U.S. West Texas Intermediate (WTI) crude futures were at $49.37 per barrel, down 29 cents, or 0.6 percent, from their last settlement.
Traders said the price declines were a result of higher output in conflict-torn Libya, which was adding to a relentless rise in U.S. production.
Libya's oil production is expected to rise to 800,000 barrels per day (bpd) this week, state-run National Oil Corporation said on Monday.
That would likely boost its exports. Shipping data in Thomson Reuters Eikon shows that, excluding pipeline exports, Libya shipped out an average of 500,000 bpd of crude oil so far this year, compared with just 300,000 bpd shipped on average in 2016.
Libya's rising production and exports add to soaring U.S. output, which largely thanks to shale oil drilling has jumped by more than 10 percent since the middle of last year to over 9.3 million bpd <C-OUT-T-EIA>, close to top producers Saudi Arabia and Russia.
"Libyan and shale oil production seems to have occupied the mind of traders overnight. That's consistent with my sense that this is all about inventories and the associated supply overhang in crude oil markets at the moment," said Greg McKenna, chief market strategist at futures brokerage AxiTrader.
Rising output from the United States and Libya undermines efforts by the Organization of the Petroleum Exporting Countries (OPEC) and other producers including Russia to tighten an oversupplied market by cutting production by around 1.8 million bpd until the end of the first quarter of 2018.
The cuts, which have been in place since January and were initially due expire in June, have so far not had the desired effect of substantially drawing down excess inventories.
Libya is an OPEC member, but it was exempt from the cuts. The United States is not participating in the self-imposed production cuts.
(Reporting by Henning Gloystein; Editing by Richard Pullin)