Oil dropped on Monday as signs of a growing glut in refined products outweighed a fall in Saudi crude exports and slower U.S. rig activity.
Crude prices have fallen for three weeks in a row on expectations of increased oil sales from Iran following a deal to ease sanctions against the OPEC producer.
Brent crude for September was down 45 cents at $56.65 a barrel by 1330 GMT. The benchmark fell nearly 3 percent last week and is down more than 10 percent so far this month.
U.S. crude futures for August were down 27 cents at $50.62 a barrel. The August contract expires on Tuesday.
The dollar’s strengthening added further pressure as it makes dollar-priced commodities more expensive for investors using other currencies.
Saudi Arabia’s crude exports fell in May to their lowest since December, with official data showing daily shipments at 6.935 million barrels per day (bpd) compared with 7.737 million bpd in April, despite record-high output of over 10 million bpd.