Oil prices fell Thursday ahead of the release of a U.S. government report expected to show U.S. crude and gasoline supplies rose last week.
Light, sweet crude for July delivery dropped 13 cents to US$63.36 a barrel in Asian electronic trading on the New York Mercantile Exchange, midmorning in Singapore. The contract on Wednesday rose 34 cents to settle at US$63.49 a barrel.
There were no clear drivers for Thursday's decline in crude futures.
"It seems some traders are closing positions as it's the end of the month," said Hitoshi Inagawa, a Tokyo-based trader with Yutaka Shoji.
The U.S. Energy Department's weekly inventory report - to be released later Thursday - will weigh heavily on energy markets. It is forecast to show domestic gasoline stocks rose 1 million barrels for the week ended May 25, according to a Dow Jones Newswires poll of analysts.
Crude oil inventories were expected to climb about 300,000 barrels, and distillates stocks were predicted to rise about 500,000 barrels, according to the poll.
July Brent crude lost 4 cents to US$67.80 a barrel on the ICE Futures exchange in London, after tumbling 50 cents overnight to settle at US$67.84 a barrel.
News on Tuesday that Valero Energy Corp. restarted refineries in Delaware and Texas was tempered by reports BP PLC was delaying the restart of a refinery in Toledo, Ohio, and that Sunoco Inc. had shut down part of a New Jersey refinery.
Markets were unsettled by news that protesters in Nigeria had besieged a major pipeline center feeding a Royal Dutch Shell PLC oil export terminal in Nigeria, forcing the company to suspend temporarily exports of 150,000 barrels of crude oil a day.
Also in Nigeria, four American oil workers, subcontractors for Chevron Corp., were released Wednesday. They were taken hostage weeks ago in the restive southern oil heartland.
Growing lawlessness, including armed attacks, in the oil-rich southern Niger Delta has cut about a quarter of the normal exports of Africa's leading oil producer. Some 200 foreign workers have been kidnapped - most quickly released for ransom - since armed militants stepped up their attacks against the oil companies and government in late 2005.
Some 20 foreign hostages remain in captivity.