Oil prices broke a four-day winning streak on Friday as dealers cashed in their profits after embarking on crude's steep climb.
Crude shot up in the past several sessions on concerns over low supply, as well as a pledge by the Opec cartel to further tighten global stockpiles this spring, dealers said.
US light crude oil futures on the New York Mercantile Exchange settled down 40 cents at $35.60 a barrel. Still, prices are up more than $3 from the day Opec announced new output cuts.
London Brent closed down 13 cents at $30.69 a barrel.
NYMEX oil dealers said traders were squaring their books ahead of the March contract's expiry at the end of the session Friday, bucking the recent bullish trend.
"People are focused on the March crude, with traders still unwinding positions," said a NYMEX floor trader.
One analyst noted that the March contract's open interest was still above 50,000 lots at midweek, a large number just before expiration and a likely factor in the sell-off.
Oil prices had been sizzling since February 10, when Opec pledged to rein in 1.5 million barrels per day (bpd) of supply its members have been producing over their quotas, and cut another 1 million bpd from the official quota starting in April.
Opec, which controls about half of the world's exported oil, is concerned that an expected downturn in global demand in the second quarter will undercut prices.
Analysts said Opec's decision to cut output showed its determination to stop supplies swelling, even though traders have reported no sign yet of any of the cartel members reducing sales, with March crude allocations unchanged from February.
"Opec has declared the extent of the first tranche of oil that it is prepared to cut in order to defend prices, and has signalled that more cuts will be made if necessary," said Barclays Capital in a report.
The strict Opec policy comes against the backdrop of tight supply in the United States, the world's largest oil consumer, where stockpiles are near 28-year lows.
The US government reported on Thursday an increase of 4.9 million barrels in crude stocks last week, with a large concentration of the build on the US West Coast, which is seen as disconnected from the big markets to the east.
A drop in crude stocks in the Midwest region, which includes the Cushing, Oklahoma, delivery point for oil traded on the New York Mercantile Exchange, was more significant for the price, analysts said.
"Despite this large crude build, there was actually a draw of 1.1 million barrels in PADD 2 (the Midwest), which is what may be getting our price up," said Katherine Spector, energy strategist at Deutsche Bank.