Oil prices extended their freefall on Friday, flirting with 11-year lows, after the International Energy Agency (IEA) warned that global oversupply of crude could worsen next year. Brent and U.S. crude's West Texas Intermediate (WTI) futures fell as much as 5 percent on the day and 12 percent on the week as mild pre-winter weather and a plummeting U.S. stock market added to the toll on oil prices. Oil traders and analysts alike have been perplexed by oil's decline since the Dec. 4 meeting of the Organization of the Petroleum Exporting Countries which all but abandoned price support for crude by removing OPEC's production ceiling in an oversupplied market. "It's very tough to find a cause to get bullish here," said Peter Donovan, broker at Liquidity Energy in New York.