Oil dropped to $66 a barrel on Thursday (August 19).
That's its lowest since May, amid concerns about weaker demand as cases of the Delta variant rise.
A stronger U.S. dollar and a surprise increase in U.S. gasoline inventories has also weighed.
Virus-related deaths have spiked in the United States over the past month.
Analysts say that's making investors cautious and pragmatic, leading to gradually softer prices.
In morning trade, Brent crude was down over 3 percent.
While U.S. West Intermediate fell as much as 3.5%.
Both Brent and U.S. crude have declined for six days in a row.
The International Energy Agency last week trimmed its oil demand outlook due to the spread of the Delta variant.
OPEC, however, left its demand forecasts unchanged.
The strong U.S. dollar is adding to the pressure.
It's rallied on expectations the Federal Reserve will start tapering its stimulus this year.
A strong dollar makes oil more expensive to other currency holders and tends to weigh on prices.