US oil settles down 2.8%, at $41.75 a barrel; worst since Aug.

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Crude prices tumbled to 2½-month lows on Thursday after the U.S. government reported a stockpile build four times above market expectations.

Also weighing on markets was an OPEC forecast that said maintaining the producer group's current pumping rates would boost supplies by half a million barrels of crude daily by next year.

Crude futures were down about 3 percent each after U.S. oil inventories rose for a seventh straight week. Stockpiles increased by 4.2 million barrels last week, compared with expectations for a million-barrel build in a Reuters poll.

"It's another data point highlighting the oil glut in the U.S. or the global markets for that matter," said Chris Jarvis, analyst at Caprock Risk Management in Frederick, Maryland.

Read MoreTexas jobs, consumer economy feel the low price of crude oil

U.S. crude futures closed down 2.75 percent, at $41.75 a barrel, the lowest settlement since Aug. 26. Brent crude futures were down $1.80 at $44.10.

A pump jack and pipes at an oil field near Bakersfield, California.
Lucy Nicholson | Reuters

Crude futures have fallen in six of the last seven sessions, losing more than $5 a barrel.

Gasoline stocks, which have been hovering at seasonal record highs since early October, fell by 2.1 million barrels, compared with analysts' expectations in a Reuters poll for a 807,000 barrels drop.

Distillate stockpiles, which include diesel and heating oil, rose by 352,000 barrels, versus expectations for a 931,000 barrels drop, the EIA data showed. Inventories now stand at their highest for this time of year since 2010.

Read More Average US gasoline price edges higher, ends 19-weeks of losses: Lundberg

Sentiment has also been hit by a growing sense that Asia's two biggest economies were slowing sharply after China's factory output growth eased further.

The slowdown in China has pulled down the entire commodity sector, with products such as crude, copper, liquefied natural gas, coal, and iron ore down between 20 and 30 percent this year.

Adding to demand worries are fears that Japan's economy may have fallen into recession and that emerging markets across the world are struggling with a debt mountain that threatens growth.