At around 16.45 GMT, US benchmark West Texas Intermediate for delivery in October recoiled USD 1.55 to USD 44.80 per barrel.
Brent North Sea crude for the same month lost a hefty USD 1.30 to USD 47.07 a barrel compared with the closing level yesterday.
The US government's Department of Energy (DoE) reported that commercial crude inventories rose by 2.3 million barrels in the week to August 26.
That easily outstripped market expectations for a smaller gain of 1.3 million barrels, according to analysts polled by Bloomberg News, and followed a 2.5-million gain the previous week.
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"Oil prices added to early declines after DoE inventory data showed a build," said CMC Markets analyst Jasper Lawler.
Rising US stockpiles tend to send oil prices lower because they add to the global supply glut - and indicate weakening demand in the world's top consumer of crude.
The DoE added today that gasoline or petrol reserves fell 700,000 barrels, while distillates - including diesel and heating fuel - rose 1.5 million.
Prices also fell on the back of the stronger greenback, which makes dollar-denominated commodities more expensive for buyers using weaker currencies.
The comments by Bijan Zangeneh, carried by Iran's official news agency yesterday, added to a feeling that an informal OPEC meeting with Russia in September may not result in a deal to boost prices.
Zangeneh said Iran needed to raise its output to regain the market share lost while it was under international sanctions, which were lifted only in January.
Iran has struggled to raise production above four million barrels per day since the sanctions were removed, according to the report, which said it is currently producing 3.8 million.