Brent North Sea crude for delivery in August gained 12 cents to stand at $106.14 a barrel in late London trading.
New York's main contract, light sweet crude for July, fell 14 cents at $98.30 a barrel.
The US Department of Energy on Wednesday said that American crude reserves last week climbed by 300,000 barrels to 394.1 million barrels. Analysts' consensus forecast had been for a drop of 400,000 barrels, according to a survey by Dow Jones Newswires.
The United States is the world's largest consumer of crude oil, though Wednesday's data added to fears of falls in demand amid the country's modest economic recovery.
Later Wednesday, the US Fed's Federal Open Market Committee will issue a statement after a two-day policy meeting, which will be quickly followed by a briefing from Fed chairman Ben Bernanke.
Investors have been speculating about when the Fed will start tapering off its $85 billion-a-month bond-buying programme, known as quantitative easing and which is credited with helping push global equities to recent highs.
Traders were keeping an eye on also the civil war in Syria, concerned that it could worsen and push the crude-rich Middle East region into a wider conflict.
Leaders from the influential Group of Eight nations on Tuesday called for a peace conference for the country and agreed to push for a transitional government that could include defectors from President Bashar al-Assad's regime.