West Texas Intermediate futures for September delivery dropped to $41.97 (-1.25%), while Brent crude fell to $48.60 (-1.20%) after the latest data had shown the number of drilling rigs in the U.S. increased last week.

In addition to oversupply issues the devaluation of the yuan by China's central bank led to the weakening of other emerging market currencies thus making imports of oil more expensive for corresponding economies.

Citigroup Inc lowered its crude oil price outlook citing such negative factors as robust supply from OPEC and doubtful demand growth in Asia as China's economic growth slows and Japan GDP contracts.