Data from market intelligence firm Genscape showed stockpiles at the Cushing, Oklahoma delivery point for U.S. crude futures rose by more than 1.8 million barrels between Oct 30 and Nov. 5, traders who saw the data said.
On Wall Street, the key U.S. stock gauge, the S&P 500, was down more than 1 percent.
Oil prices were up earlier in the session after OPEC said it expected global demand to remain strong next year. Abdullah al-Badri, Secretary-General of the Organization of the Petroleum Exporting Countries, said he expects the oil market will become more balanced in 2016 as demand continues to grow.
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Global oil demand growth is set to reach a five-year high in 2015 at 1.8 million barrels per day, but is expected to slow down in 2016, the International Energy Agency said last month.
OPEC will meet on Dec. 4 to discuss the cartel's strategy after last year it opted to maintain its output steady in the face of lower prices and rising U.S. shale production.
On Monday, United Arab Emirates, one of the wealthiest Gulf states, said it is pushing ahead with large new energy projects, betting an oil price recovery will start as early as next year as demand begins to absorb the global glut.
Hamza Khan, commodities analyst at ING Bank in the Netherlands, said any upside for oil is limited as oil producers around the world struggle to sell a surplus.
"It will be very difficult to see upside traction before the meeting in case OPEC announce they will maintain production at current levels," he said. "Any movements without fundamental support will be limited, specially on the upside."
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The dollar was down 0.19 percent versus a basket of currencies. The dollar index hit a near seven-month high on Friday following strong U.S. jobs data that increased the likelihood of a rate increase by the Federal Reserve.
A stronger greenback makes dollar-priced assets more costly for buyers using other currencies.
China's trade surplus rose to a record of $61.64 billion in October, disappointing analyst expectations by a wide margin, driving up concerns over growth in the world's top energy consumer as the country's sector service also slowed.
"The market continues to look fragile and prone to lower numbers," brokerage PVM said in a report.