Light, sweet crude for November delivery recently was trading 65 cents, or 0.8%, lower at $79.29 a barrel. Brent crude on the ICE futures exchange fell 39 cents to $76.80 a barrel.

"The surprise inventory build put a little pressure on (crude prices)," said Tom Bentz, director and senior analyst at BNP Paribas Commodity Futures. The market is in a lull at the start of refinery turnaround season, which usually results in a dip in crude demand, Mr. Bentz said. "There could be some downward pressure here in the short term," he added.

Gasoline and distillate inventories posted unexpected declines last week.

Front-month November reformulated-gasoline blendstock, or RBOB, was down 83 points, or 0.4%, at $1.9876 a gallon. November heating oil was down 1.58 cents, or 0.7%, at $2.1629 a gallon.

The 300,000-barrel increase in stocks at Cushing, Okla., the delivery point for Nymex crude, is putting pressure on the front-month spread, the difference between the prices of the front-month contract and the second-month contract, said Tony Rosado of IAG Energy Futures in Ft. Lauderdale, Fla.

"The crude market has been holding a tight range, which tells us the market can still work higher," mr. Rosado said. "The builds were light, which tells us there's a certain tightness in the marketplace."

The unexpected increase in crude oil inventories hasn't snapped futures' upward momentum, traders said.

"These (inventory) numbers show a consistency in demand that means it will only be a matter of time before this market resumes its upward trajectory," said Phil Flynn, senior market analyst for Alaron Trading Corp. in Chicago, in a note. "At the first sign that seasonal demand kicks in, we should see oil again test the highs and, more than likely, exceed them."

The dollar, which has rebounded slightly from recent record lows against the euro this week, is still around $1.41 to the euro. Analysts are split on whether dollar volatility has played a key role in crude's swing past $80 and current stagnation.