Oil prices fell more than a dollar Wednesday after the US government reported a surprisingly large rise in crude supplies in the world's biggest energy consumer.
Light crude on the New York Mercantile Exchange fell $1.07 to $33.03 a barrel, while London Brent futures slipped 55 cents to $28.95 a barrel.
The losses came after the US Energy Information Administration reported a 7.9 million barrel increase in commercial crude inventories last week caused by high imports and low domestic refinery activity.
The market had expected crude supplies to rebound from a 28-year low, but only by roughly 600,000 barrels, a Reuters survey showed.
Inventories are still 28.9 million barrels below the 5-year average for this time of year, but dealers anticipated the data could signal a building trend for stocks.
"It's not surprising that there was a crude build since refiners are reaching peak of maintenance and their demand was falling off," said Aaron Brady, senior oil analyst at Energy Security Analysis Incorporated (ESAI).
Crude imports averaged nearly 10.5 million barrels per day (bpd) last week, up almost 1.9 million bpd from the previous week, the EIA reported.
Refinery utilisation, meanwhile, fell 1.2 percentage points to 87.6 percent last week as plants shut down processing units for planned winter maintenance.
Experts have said the planned work could reduce refiner demand for crude oil by about 1 million bpd in February, allowing tight stockpiles to replenish.
The losses Wednesday brought oil prices down around $2 from 10-month highs hit last month. And the end is nearing for the US winter heating season.
Tempering the losses, however, were rising concerns over gasoline supply ahead of spring due to new fuel standards that could be more difficult for refiners to meet.
Dealers have also been eyeing signals from Opec members on whether the cartel might adjust its output policy to bring the price of its reference basket of crudes back down into its target range.
The group, which controls 40 percent of the world's oil exports, is scheduled to meet in Algiers on February 10.
Several Opec ministers have said in recent days that the cartel is likely to agree to leave its official output ceiling of 24.5 million bpd unchanged.
But the group, worried that world oil demand will fall in the second quarter as rising temperatures in the northern hemisphere reduce oil demand, plans to meet again on March 31.
A Reuters survey on Wednesday showed the 10 members of Opec with quotas overproduced in January by more than 1.75 million bpd.
Leading Opec producer Saudi Arabia added 80,000 bpd in January to hit 8.55 million bpd - nearly 600,000 bpd above its quota. Saudi Arabia was the only member to implement cuts Opec agreed when it reduced its official production ceiling by 900,000 bpd from November 1.