Brent crude futures fell 64 cents, or 1.2 percent, to $50.82 a barrel by 12:52 p.m. EDT (1652 GMT).
US West Texas Intermediate (WTI) crude futures fell 60 cents, or 1.1 percent, to $49.92.
Trade group American Petroleum Institute will issue at 4:30 p.m. EDT (2030 GMT) a weekly report of crude stockpiles and other oil supply-demand data, ahead of an official report by the U.S. government`s Energy Information Administration on Wednesday.
Analysts polled by Reuters expected the data would show crude stocks rose 800,000 barrels last week, after a drop of more than 5 million barrels in the week to Oct. 14.
"The sentiment is it`s a bit more negative," said Scott Shelton, energy futures broker with ICAP in Durham, North Carolina. "There are some expectations that we can see a crude build."
A rallying dollar, which makes greenback-denominated crude less affordable to holders other currencies weighed further on oil, as did falling Wall Street share prices.
The market is also "a bit rubbery in the knees" due to uncertainty around OPEC`s planned production cut, said Donald Morton, who runs an energy-trading desk at Herbert J. Sims & Co in Fairfield, Connecticut.
Before this week, prices rose about 13 percent over a three-week span after OPEC announced on Sept 27 its first planned output cut in eight years to combat the steep slump in crude prices, which are far below 2014 highs above $100 a barrel.
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