A forecast by the International Energy Agency (IEA) for slower world oil demand next year was also weighing on the market, analysts said.
US benchmark West Texas Intermediate for August delivery was down 86 cents to $51.88 and Brent crude tumbled 96 cents to $57.77 a barrel in late-morning trade.
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"I hope we are finally entering the final phase of these marathon negotiations. I believe it," said French Foreign Minister Laurent Fabius, who cancelled a trip to Africa to stay at the talks in Vienna.
Any deal to curb what the West suspects as Iranian efforts to build an atomic bomb will result in the lifting of punishing economic sanctions, allowing the country to resume oil exports.
More Iranian oil however will add to a supply glut, which has depressed prices.
"Reports out this morning have suggested that a comprehensive 'in-principle' agreement has been reached over the main issues, with only technical considerations remaining unresolved," said Nicholas Teo, market analyst at CMC Markets in Singapore.
"This may once again move the needle for crude," he said in a market commentary.
The IEA has forecast that global oil demand would grow by 1.2 million barrels per day next year, slower than the 1.4 million projected this year.
However, global output grew by 550,000 barrels a day in June alone to 96.6 million barrels, IEA added.
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