Oil prices fell more than 1 percent on Monday because of strong U.S. data and concerns over the efficiency of the production cuts of OPEC.
Brent futures were down 60 cents at $51,16 per barrel.
WTI futures were trading 71 cents lower at $48.07 per barrel.
"The continued growth of U.S. rigs reinforces the bearish sentiment," said Carsten Fritsch, senior analyst at Commerzbank in Frankfurt.
United States drillers added 14 oil rigs last week, the total number of which amounted to 631, according to Baker Hughes Inc data on Friday.
The prospect of higher oil supplies from Libya will further aggravate the bearish sentiment.
Libya's National Oil Corporation (NOC) said that it was confident in regaining control of two key oil ports of Es Sidra and Ras lanuf, which have a combined capacity to export 600,000 barrels per day.