At about 1615 GMT, US benchmark West Texas Intermediate (WTI) for delivery in June lost USD 1.35 to USD 43.43 per barrel.
Brent North Sea crude for July delivery shed 95 cents to USD 44.88 a barrel compared with yesterday's close.
"This may be a dreaded month for the equity market investors ... But crude oil is having an equally frightening start too," said City Index analyst Fawad Razaqzada.
"Oil prices have also been absorbing a lot of negative news lately - and completely ignoring the fact the markets remain oversupplied with US crude oil inventories being at record-high levels.
So-called black gold has rallied for the last four weeks thanks to a weaker dollar and hopes that the world economy is slowly turning positive. A faltering greenback makes dollar-priced crude cheaper for buyers using stronger currencies, which tends to stimulate demand and prices.
However, the oil market fell yesterday as figures showed Iraqi exports hit a near-record 3.36 million barrels a day in April, while Iran's production reached 3.5 million barrels per day, the most since December 2011.
"Fundamentally, there are no changes (in the supply and demand balance). In fact the situation seems to be getting worse," added IG analyst Bernard Aw.
He said that while the recent rebound seemed "intact", it would be a challenge for prices to return to USD 50 a barrel unless the Organisation of Petroleum Exporting Countries (OPEC) takes firm action to ease the oversupply during its twice-yearly meeting next month.
