Benchmark West Texas Intermediate crude for December delivery fell 81 cents to US$93.03 a barrel on the New York Mercantile Exchange.
Oil has traded between $93 and $96 a barrel over the past two weeks and is down from nearly $110 a barrel in early October.
Iran will resume talks this week in Geneva with six world powers — the U.S., Britain, France, Russia, China and Germany — aimed at resolving a decade-long standoff over Iran's nuclear program.
The powers are offering a gradual rollback of sanctions that have crippled Iran's economy, raising concerns of an influx of Iranian oil into world markets at a time of already abundant supplies.
However, two U.S. administrative officials said last week that Iran would get only limited and temporary relief from economic sanctions.
Brent crude, the benchmark for an international variety of crude, slipped three cents to US$108.47 a barrel on the ICE Futures exchange in London.
In other energy futures trading on Nymex, wholesale gasoline was flat at US$2.66 a U.S. gallon (3.79 litres), heating oil was down two cents at US$2.92 a gallon and natural gas dropped four cents to US$3.62 per 1,000 cubic feet.
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