US benchmark West Texas Intermediate for September delivery fell 86 cents to USD 47.66 a barrel.
Brent North Sea crude for September shed 62 cents to USD 52.69 a barrel in afternoon London deals.
Both contracts tumbled yesterday, snapping two consecutive days of gains.
The heavy drop came after Abdullah El-Badri, secretary- general of the Organisation of Petroleum Exporting Countries, said the group would not cut output in response to lower prices.
Speaking in Moscow after meeting Russia's energy minister yesterday, he said the cartel is "not ready" to cut production, which is currently at around 30 million barrels per day.
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"OPEC is telling the market that cuts will not come from them," said Daniel Ang, an investment analyst with Phillip Futures in Singapore.
OPEC is "emphasising that it is fighting for market share", he added.
At its most recent meeting in Vienna in June, OPEC kept its output levels despite the supply glut, which has depressed oil prices.
Crude futures are under pressure also owing to the strength of the US currency, which makes dollar-priced oil more expensive to holders of weaker units, dampening demand.
The chances of a September lift were raised yesterday after data showed the US economy expanded 2.3 per cent in the April-June period, the strongest pace since the third quarter of 2014.
"The second-quarter GDP data support the Fed's more upbeat tone on economic conditions and suggests that the economy could cope with higher interest rates," research firm Capital Economics said.