Benchmark West Texas Intermediate crude for June delivery was down $2.60, or 2.7 per cent, at $93.79 a barrel in morning trading on the New York Mercantile Exchange.
Since oil is traded in dollars, a stronger U.S. currency makes crude and other commodities less appealing for investors with other currencies.
On Friday, the euro was down to US$1.2947 from $1.3041 late Thursday in New York. The dollar also gained on the Japanese yen and was trading at 101.92 yen on Friday, after breaking above 100 yen on Thursday for the first time since April 2009.
The Organization of Petroleum Exporting Countries said it produced 30.5 million barrels of oil a day in April, the most since November. The group said global supplies increased 450,000 barrels a day. Production from non-OPEC nations like the U.S. is also on the rise at a time when supplies are ample and mixed global economic data raise questions about demand.
OPEC said it expected global oil use to be "much higher" in the second half of the year, but warned that the struggles of the eurozone and possible lower growth in China could hurt demand.
"A fragile recovery in the global economy has been visible since the beginning of the year, but momentum has started slowing again and growth risks are skewed to the downside," OPEC said in its monthly report.
Brent crude, a benchmark for many international oil varieties, was down $2.41, or 2.3 per cent, to US$102.06 a barrel on the ICE Futures exchange in London.
In other energy futures trading on the Nymex, wholesale gasoline fell six cents to US$2.83 a U.S. gallon (3.79 litres), heating oil lost seven cents to US$2.86 a gallon and natural gas fell one cent to US$3.96 per 1,000 cubic feet.
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