Brent futures had declined 42 cents to $47.93 a barrel by 06:43 GMT, after closing down 37 cents in the previous session, according to Reuters.
U.S. crude futures dropped 32 cents to $47.76 a barrel, having settled down 33 cents the day before.
Both contracts had finished with modest losses for a fourth straight session.
U.S. commercial crude oil stocks likely fell by around 2.5 million barrels to 538.8 million in the week ended May 20, a preliminary Reuters analysts' poll taken ahead of weekly industry and official inventory data showed on Monday.
Read alsoReuters: Iran has no plans to freeze oil exportsGasoline stocks probably dropped 1.3 million barrels last week, while distillate inventories, which include heating oil and diesel fuel, likely decreased by a million barrels, the poll showed. The United States is gearing up for its summer driving season.
The American Petroleum Institute (API) is due to release inventory data on Tuesday, while figures from the U.S. Department of Energy's Energy Information Administration (EIA) will come on Wednesday.
