Hedge funds have slashed their bets on oil prices to their most bearish in 20 weeks, throwing up a big hurdle to Opec's efforts to support prices as it prepares to meet later this month.

Data from ICE and the U.S. Commodity Futures Trading Commission showed that money managers cut their net long positions on Brent and WTI crude by 18,829 to 252,261 in the week to Nov. 14, the lowest level since June 27.

Oil prices snapped a fourth straight week of losses on Friday as weak trade data and fresh doubts over whether the Federal Reserve has finished raising interest rates clouded the outlook for demand. At the same time, supply has been exceeding expectations in recent weeks, leading to a steady weakening of real crude prices.

Crude oil prices rebounded on Friday on growing expectations that Opec and its Allies will try to offset recent rout at their next meeting on Nov. 26. As the front-month contract nears expiration, the volume of WTI crude oil is reduced and there is also potential for high volatility.