West Texas Intermediate futures for August delivery dropped to $51.37 (-2.60%); Brent crude declined to $56.93 (-3.06%) as markets anticipated Iran and global powers to make a deal, which would pour more oil into oversupplied markets. However additional exports in the form of 500,000 to 700,000 barrels a day is unlikely before late 2015 or early 2016.

Data from China showed Monday that the country's crude imports in the first half of the current year rose 7.5% from a year ago, but that failed to support prices because the increase was mostly led by stockpiles of strategic reserves rather than by a real rise in demand.

Several banks have lowered their oil price forecasts amid excessive supplies.