Benchmark crude for June delivery was down 47 cents to USD 96.24 a barrel in midday trading on the New York Mercantile Exchange.
On Wednesday, Fed chairman Ben Bernanke will appear before Congress and the central bank will release minutes of its most recent policy meeting.
Traders will be looking for hints on what the Fed might be preparing to do in light of recent data that has pointed toward a sustained economic recovery.
There is speculation that the Fed might want to scale back or modify its loose monetary policy and its USD 85 billion-a-month programme of bond purchases intended to keep interest rates low and prop up the recovery.
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Data for the week ending May 17 is expected to show draws of 1.2 million barrels in crude oil stocks and of 200,000 barrels in gasoline stocks, according to a survey of analysts by Platts, the energy information arm of McGraw-Hill Cos.
The American Petroleum Institute will release its report on oil stocks later today, while the report from the Energy Department's Energy Information Administration, the market benchmark will be out tomorrow.
Brent crude, a benchmark for many international oil varieties, was down 68 cents to USD 104.12 a barrel on the ICE Futures exchange in London.