The American Petroleum Institute reported a 2.2 million barrel rise in reserves last week, confounding expectations of a dip at the height of the crucial summer driving season when Americans go on holiday.
The figures rattled traders as the Department of Energy prepares to release later Wednesday official data, which is considered a key gauge of demand in the world's top oil consumer.
At about 0340 GMT, US benchmark West Texas Intermediate fell 27 cents, or 0.58 per cent, to USD 46.53 while Brent was 36 cents, or 0.74 per cent, lower at USD 48.11.
"But of course, we need to see whether this will be a one-time gain or not."
Prices for both contracts surged almost five percent Tuesday from two-month lows after OPEC said it expected the global supply glut to ease this year and next thanks to reductions in output from producers outside the cartel, particularly the United States.
Prices have fluctuated between $44 and $52 a barrel over the past month after hitting near 13-year lows below $30 in February, but a surge in OPEC production in June revived fears about oversupply.
This is due to a tentative recovery in production in Nigeria and Libya and a "steady" rise in output from Saudi Arabia and Iran, it said.
"We had the bulls for a couple of weeks and the bears are starting to step in," David Lennox, an analyst at Fat Prophets in Sydney, said.
"The market has realised that surplus that we had was disrupted, and that it is coming back because those disruptions are going away."
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