Brent crude oil prices fell below $49 a barrel and U.S. crude also fell more than $1 on Monday after the global economic outlook darkened and Iraq announced record oil production.
The world's biggest energy consumer, China, faces significant downward pressure on its economy, its premier Li Keqiang was quoted by state radio as saying on Monday.
China is expected this week to report growth slowing to 7.2 percent from a year ago, the weakest since the depths of the last global economic crisis. Data from China's National Bureau of Statistics showed on Sunday house prices fell for a fourth straight month.
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Brent crude settled at $48.84 a barrel, down $1.33.
U.S. crude was last trading down $1.17 at $47.52 a barrel.
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Trading volumes were thin and there was no official settle for U.S. crude as U.S. markets were closed for the Martin Luther King Jr. Day holiday.
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Oil prices have dropped by more than half since June as output around the world soared while demand growth slowed.
Although the International Energy Agency (IEA) said last week a reversal in the trend was possible this year, it noted prices may fall further before rising.