Oil extended losses below $50 a barrel amid speculation that U.S. inventories will expand, deepening a global supply glut that's driven prices to a five-year low.

Futures declined for a fourth day. Stockpiles in the world's biggest oil consumer probably rose by 700,000 barrels last week, a Bloomberg News survey shows before a government report tomorrow.

Oil slumped almost 50 percent in 2014, the most since the 2008 financial crisis, after the Organization of Petroleum Exporting Countries resisted calls to cut output as it competes with U.S. producers. The market faces "more problems" this year, according to Morgan Stanley, with surging output in Russia and Iraq contributing to a surplus that Qatar estimates at 2 million barrels a day.

"The path of least resistance is lower," said Bill O'Grady, chief market strategist at Confluence Investment Management in St. Louis, which oversees $2.4 billion. "There is no bullish news. OPEC refuses to cut production and there is no evidence of falling production outside of OPEC."

West Texas Intermediate for February delivery dropped 98 cents, or 2 percent, to $49.06 a barrel at 10:06 a.m. on the New York Mercantile Exchange after touching $48.47, the lowest since April 2009. The volume of all futures traded was about 83 percent above the 100-day average for the time of day.

Brent for February settlement decreased 70 cents, or 1.3 percent, to $52.41 on the London-based ICE Futures Europe exchange after reaching $51.23, the lowest since May 2009. The European benchmark crude traded at a premium of $3.33 to WTI on the ICE.