US benchmark West Texas Intermediate (WTI) for August delivery closed at $52.20 a barrel in New York trade, down 54 cents from Friday’s settlement.
Brent North Sea crude for August shed 88 cents at $57.86 in London.
“The crude oil market is on the defensive in Monday trade as investors sort through the headlines since Friday,” said Tim Evans of Citi Futures.
Marathon talks between six major powers and Iran over Tehran’s nuclear program were making “real progress” but issues remained, the United States said Monday, suggesting the Vienna talks would stretch into another day.
A deal could result in the lifting of sanctions that have sharply curbed Iran’s oil exports.
The oil market is “wary that an Iranian nuclear deal could mean more supply, although it won’t be right away,” said Evans.
Greece’s bailout deal meanwhile lifted the dollar, which makes dollar-priced crude oil less attractive to buyers using other currencies.
“Oil traders are more concerned that the euro is weaker and the dollar stronger on the news,” said Evans.
Meanwhile in its July monthly report, OPEC raised its forecast for growth in crude demand this year by 100,000 barrels to 1.28 million barrels per day.
In 2016, demand is expected to pick up by 1.34 million barrels per day, hitting 93.94 million barrels per day, thanks to stronger economic growth, according to the OPEC report.
“OPEC raised its demand forecast… but with the stout supply picture, the market still has a bearish sentiment to price,” said Brian Swan, commodity analyst at Schneider Electric.