Oil prices fell on Aug. 23, weighed down by concerns about rising production from Libya feeding into an oversupplied market and a surprise increase in U.S. gasoline inventories.

Benchmark Brent crude futures were down 27 cents at $51.60 a barrel (bbl) at 4:50 a.m. CST (9:50 GMT). U.S. West Texas Intermediate crude futures were trading at $47.67/bbl, down 16 cents.

Production from Libya's Sharara oil field, the conflict-riven country's largest, has been seesawing. The field remained shut on Aug. 23, two Libyan oil sources told Reuters. The field had restarted at least once on Aug. 22 amid conflicting reports about whether it had reopened.

"[The] flood of news reports makes it clear that the situation in Libya is still chaotic and that conditions in the country are still far from normal," Commerzbank analysts wrote.

Sharara recently reached an output of 280,000 bbl/d, but closed this week due to a pipeline blockade. Its production is key to Libya's oil output, which surged above 1 million bbl/d in late June, about four times its level last summer.

Libya's rising output is a headache for OPEC, which together with non-OPEC producers including Russia has pledged to cut around 1.8 million bbl/d of supplies between January this year and March 2018 in an attempt to remove a global glut.

Additionally, industry data released by the American Petroleum Institute showed on Aug. 22 that U.S. gasoline stocks rose by 1.4 million bbl in the week to Aug. 18, compared with analysts' expectations of a 3.5-million-bbl drop.

Jeffrey Halley, senior market analyst at futures brokerage OANDA, said rising U.S. gasoline inventories were "not a good sign during the U.S. summer driving season."

Official inventory data from the U.S. Energy Information Administration is due later on Aug. 23.