Crude oil prices fell further into the red in late trading after a private report showed that oil inventories spiked last week.
West Texas Intermediate, the US oil benchmark, was down to $51.73 a barrel, after settling earlier in the afternoon at $52.17. It had ended last week at $53.83.
The latest leg lower came after on the heels of bearish inventories data from the American Petroleum Institute. US oil stocks jumped by 14.2m barrels last week, compared with analyst estimates of 2.5m barrels, according to a Reuters report of the API data.
US government data on oil inventories are due out on Wednesday morning.
The sharp rise in stocks compounds concern among some analysts that with oil prices hovering above the $50-a-barrel level, US energy groups will begin to pump more aggressively, offsetting the production cut that was forged between Opec countries and several other large exporters, like Russia.