US oil prices dropped for the fifth straight trading session Wednesday as domestic crude stockpiles rose again, but London prices inched higher.

US benchmark West Texas Intermediate for August delivery fell 68 cents to $51.65 a barrel.

Brent North Sea crude for August climbed 20 cents to settle at $57.05 a barrel in London trade.

Markets were still on edge over global geopolitical issues, including the meltdown in Chinese stocks, the Greek debt crisis and the crunch talks on Iran's nuclear program, all of which could impact the global supply-demand balance.

Meanwhile US commercial crude stockpiles rose another 400,000 barrels to 465.8 million barrels in the week to July 3, even as refineries picked up activity. Gasoline inventories also jumped, with the summer holiday driving season in full swing.

Nevertheless, Tim Evans of Citi Futures said prices were being supported "by a weaker US dollar and bargain hunting following the recent price drop."

"However, we note supplies remain well above year-ago levels, consistent with an ongoing global supply/demand surplus."