"US inventories saw another surprise build, adding to concerns of oversupply," noted CMC Markets analyst Jasper Lawler.
At about 1600 GMT, US benchmark West Texas Intermediate for September delivery slid 83 cents to USD 41.94 per barrel.
Brent North crude for October lost 77 cents to $44.21 a barrel compared with yesterday's close.
The US government's Department of Energy (DoE) reported that commercial crude stockpiles climbed 1.1 million barrels in the week to August 5.
That confounded market expectations for a drop of 1.5 million barrels, according to analysts polled by Bloomberg News.
The increase was however far less than the 2.1-million-barrel gain given by industry body the American Petroleum Institute (API) yesterday.
Oil prices have been fluctuating since entering a "bear" market last week, falling more than 20 per cent and closing below USD 40 a barrel for the first time since April.
They rebounded after the Organization of the Petroleum Exporting Countries (OPEC) said Monday that it would hold talks on the sidelines of the International Energy Forum in Algeria from September 26 to 28, ahead of a planned meeting due at the end of November.
Analysts said investors remained unsure about what to expect from the OPEC meeting.
The last time OPEC met in April it could not come to an agreement about the production freeze and its members have been pumping crude at record high levels.
OPEC reiterated in its August monthly report today that it expects the market to rebalance in 2017 as output from outside the group declines and demand rises.
This would be a vindication of OPEC'S Saudi-led strategy since 2014 of squeezing non-OPEC suppliers by keeping production at high levels despite low prices.