Reports that foreign ministers will review draft text of an Iran nuclear deal are "very premature and not accurate," a senior U.S. official told NBC News on Wednesday.
Earlier, U.S. crude broke below its two-month trading band earlier on reports that an Iranian diplomat said technical experts from Iran and six world powers had finished a draft agreement on the country's nuclear program and foreign ministers would review it Thursday and Friday in Vienna.
World powers and Iran held talks throughout Wednesday, extending an original June 30 deadline for a nuclear accord by a week.
"If the nuclear negotiations with Iran are brought to a positive conclusion, there is also the 'threat' of additional oil reaching the market from Iran," said Carsten Fritsch, analyst at Commerzbank.
The dollar's rally on Greece's debt default and signs of OPEC output at three-year highs further weighed on the market.
The U.S. Energy Information Administration (EIA) said crude inventories rose by 2.4 million barrels last week, the first weekly build since April. Analysts polled by Reuters forecast a 2 million-barrel draw.
The EIA also said gasoline stockpiles fell by 1.8 million barrels, indicating strong demand for fuels from the peak U.S. summer driving season, even as crude inventories rose.
"The gasoline draw implies demand has picked up. But the overall feel is that we have more than enough crude and the market could be in a bearish tilt hereon with the Greece and Iran factors in play," said Tariq Zahir, oil trader and managing member at Tyche Capital Advisors in Laurel Hollow, New York.
The dollar rose nearly half a percent against the euro, making commodities denominated in the greenback, less affordable for users of the single currency. The move came as Greece became the first developed economy to default on an International Monetary Fund loan.
OPEC supply hit a three-year high of 31.6 million barrels per day in June, a Reuters survey said on Tuesday, led by record Iraqi exports.
"With these strong growth rates, OPEC supply growth is now challenging non-OPEC supply growth," analysts at JBC Energy in Vienna wrote. "The uptick comes on the back of a massive increase in Iraqi production."
—CNBC's Patti Domm, Chris Hayes and Tom DiChristopher contributed to this report.