West Texas Intermediate futures for August delivery, which expires today, declined to $50.22 (-0.44%); Brent crude fell to $56.55 (-0.18%) amid the dollar's strength and concerns of imminent additional supplies to an already oversupplied market from Iran.

Strong dollar makes crude more expensive for holders of other currencies.

Meanwhile many analysts say that Iran cannot increase its exports significantly in the near future. "Pessimism about oil prices because of the Iran nuclear deal and economic concern about China and Europe are overblown," analysts at PIRA Energy said forecasting Iranian output rising from around 3 million barrels per day to full capacity of 3.5 million bpd by the end of 2016.

The American Petroleum Institute and the US Energy Information Administration will publish inventory reports on Tuesday and Wednesday respectively.