Oil prices fell on Friday, sliding underneath $100 per barrel in New York, as sentiment was hit by the stronger dollar and concerns over the outlook for the global economy and energy demand.
New York's main contract, West Texas Intermediate crude for delivery in February, slid 66 cents to $99.73 a barrel. The February contract expires at the close.
Brent North Sea crude for March delivery was down 41 cents at $111.14 per barrel in London midday deals.
"We had a pronounced dollar rebound this morning, but (prices fell) also on weak demand numbers reported" by the US government, said VTB Capital analyst Andrey Kryuchenkov, adding that "demand concerns" were resurfacing.
The euro hit a two-week high at $1.2986 on Friday before sliding to $1.2891 as investors took profits ahead of the weekend and eyed ongoing eurozone concerns.
A stronger greenback makes dollar-priced oil more expensive for buyers using weaker currencies, which tends to dampen demand and in turn prices.
Meanwhile on Thursday, the US Energy Information Administration revealed signs of weak oil demand in the United States.
Demand for all oil products for the four weeks to January 13 was down 7.2 percent, and 6.1 percent for gasoline.
Looking ahead, traders were awaiting the results of a meeting of European Union foreign ministers on Monday regarding plans to impose an oil embargo on major crude producer Iran.
Senior EU diplomats on Thursday agreed to sanction Iran's central bank, freezing assets used to finance its nuclear drive, but have yet to clinch an oil embargo deal.
Greece, which buys 34.2 percent of its oil from Iran, is seeking up to a year to phase out existing contracts while some countries, including Britain, France and Germany, want a three-month deadline.
A decision on an embargo is expected to be finalised when the bloc's foreign ministers meet on Monday.
The new EU sanctions are part of a concerted effort with the United States to pressure Iran into halting its controversial nuclear activities, which the West suspects are aimed at developing nuclear weapons. Iran says its nuclear programme is purely for civilian use.