Brent crude futures were down $1.41, or about 1.7%, at $83.93 a barrel at 1341 GMT. U.S. West Texas Intermediate crude was down $1.46, or about 1.8%, at $80.71. WTI fell by $2 earlier in the session, Reuters reported.

China's July oil imports were down 18.8% from the previous month to the lowest daily rate since January, but still up 17% from a year earlier.

Overall, China's imports contracted by 12.4% in July, far steeper than the expected 5% drop. Exports fell by 14.5%, compared with a fall of 12.5% tipped by economists.

Despite the gloomy data, some analysts were still positive on China's fuel demand outlook for August to early October.

The peak season for construction and manufacturing activity starts in September and gasoline consumption should benefit from summer travel demand, said CMC Markets analyst Leon Li. Demand is expected to decrease gradually after October, he added.

On the supply side, Saudi Arabia last week said it would extend a voluntary oil output cut of 1 million bpd for another month to include September, keeping the door open for further cuts by the world's biggest oil exporter.

Russia also said it would cut oil exports by 300,000 bpd in September.

The decision to extend cuts into September, despite Brent futures rising above $80 per barrel, suggests Riyadh may be targeting a higher price than $80, said Vivek Dhar, mining and energy commodities strategist at Commonwealth Bank of Australia.

Investors are also awaiting U.S. oil and fuel products inventory data.

"Overlapping, concentric and oppositional influences continue to bring nervousness to our market and oil prices will have to lean again on the state of world inventories to keep its winning ways," said John Evans, of oil broker PVM.