New York's main contract, West Texas Intermediate for delivery in December, fell 31 cents to $93.53, while Brent North Sea crude for January was also down 31 cents at $108.19.
"Iran is likely to be one of the factors," Ric Spooner, chief market analyst at CMC Markets in Sydney, told AFP.
"Also I think it's a case of a relatively subdued beginning of the week news-wise," he added. "There's a little bit of profit-taking as we head into a new week."
Negotiations between Iran and the so-called P5+1 – Britain, France, the United States, Russia and China plus Germany – restart in Geneva on Wednesday after the last round failed to seal a deal.
Top diplomats insisted they were closing in on an interim agreement that would see Iran, a huge producer of oil, curb or freeze parts of its nuclear program for some relief from crippling sanctions.
Israel and the West suspect Iran is pursuing a nuclear weapons capability alongside its uranium enrichment program, which Tehran insists is entirely for peaceful purposes
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