Light, sweet crude for August delivery lost 53 cents to US$68.61 a barrel in Asian electronic trading on the New York Mercantile Exchange, midmorning in Singapore.
The contract rose 49 cents to settle Friday at US$69.14 a barrel.
August Brent crude dropped 46 cents to US$70.72 a barrel on the ICE Futures exchange in London.
Nigerian labor unions called off their strike aimed at overturning a government fuel-price hike on Saturday, ending a four-day work stoppage. The unions said they had accepted the government's proposal to hold off on raising fuel prices for a year, while accepting an earlier proposition to halve a previous price increase that sparked the strike.
"The calling off of the Nigerian strike is the primary news item driving the oil market this morning," said Victor Shum, an energy analyst with Purvin & Gertz in Singapore, adding the strike did not seem to affect the country's oil exports. "With no disruptions, it was a threat and now that the threat has at least been temporarily removed, crude oil futures are therefore falling."
Oil receipts account for some 80 percent of Nigeria's total government revenue. Nigeria's energy industry is the biggest in Africa and the eighth-largest worldwide, and the threats to shut off the taps had earlier sent crude prices toward nine-month highs on international markets. Nymex crude futures rose Friday after talks between union leaders and the government failed to produce an agreement.
The market has been fixated on U.S. domestic supply concerns, but a lack of news from the U.S. shifted traders' focus to international developments such as Nigeria and concerns about Iran's nuclear capabilities.
Energy futures Friday also rose after Iran's interior minister, Mostafa Pourmohammadi, said his country had 220 pounds of enriched uranium. Investors' concern is that the West will at some point take action, military or economic, against Iran, which could disrupt oil supplies from the Persian Gulf.
Heating oil futures lost 1.75 cents to US$2.0205 a gallon (3.8 liters) while natural gas prices declined 12 cents to US$7.01 per 1,000 cubic feet.