Although analysts said it would take until 2016 before Iran returns to full-scale oil exports, most estimate that there would be an export jump of around 200,000 barrels per day in the short term. The global crude market already has a 2.6 million bpd surplus.
"It's all worries about supply now," said John Kilduff, partner at New York energy hedge fund Again Capital. "It just spells glut, no matter how you look at it."
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Tehran's state media said foreign ministers from Iran and six world powers will meet at 3 p.m. EDT, three hours before the latest deadline for a nuclear deal expires. Several Western officials, however, said they were unaware of such a meeting.
European Council President Donald Tusk said euro zone leaders had "unanimously reached agreement" on a deal for Athens. Greek Prime Minister Alexis Tsipras said the debt restructuring and medium-term financing deal was worth 35 billion euros ($38.7 billion).
With supply ample and economic risks growing, several banks lowered their oil price forecasts.
Bank of America Merrill Lynch said U.S. crude prices "could soon drop well below our $50 per barrel target" for the third quarter of 2015.
Commerzbank said a fall below $55 per barrel in Brent and below $50 per barrel in U.S. crude was "conceivable."
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The global oil market should be more balanced next year as China and the developing world increase oil consumption while supply of shale oil from North America and other regions grows more slowly, OPEC said on Monday.
In its monthly report, the Organization of the Petroleum Exporting Countries said it expected world oil demand to increase by 1.34 million barrels per day (bpd) in 2016, up from growth of 1.28 million bpd this year.
This would outpace the growth of oil supply from non-OPEC sources and ultra-light oils such as condensate, increasing demand for OPEC crude oil, it said.
Separately, data from the U.S. Energy Information Administration showed oil production from the largest U.S. shale plays is forecast to plunge some 72,000 barrels per day in August compared with the previous month.
Oil production from the Permian play of West Texas and New Mexico, however, is expected to rise 5,000 bpd to 2.05 million bpd, according to the EIA's drilling productivity report.
Bakken oil production is expected to fall 22,000 bpd to 1.2 million bpd, while Eagle Ford oil production is expected to drop 55,000 bpd to 1.5 million bpd.