Investing.com - Oil turned sharply lower Monday as the number of rigs operating in the U.S. continued to increase.
Brent crude shed $1.13, or 1.98%, to $55.97 at 08:00 ET. U.S. crude lost $1.04, or 1.93%, to $52.95.
An increase in North American shale activity could erode the positive impact of agreed output cuts elsewhere.
Baker Hughes U.S. rig count data Friday showed an increase of 4 rigs to 529 in the latest reporting week, the highest level in about a year.
OPEC and non-OPEC producers have agreed to cut output by some 1.8 million barrels a day from the start of this year for a period of six months.
Investors are also looking to output levels by producers exempt from the cuts such as Iran, Libya and Nigeria.
The dollar index was higher. A stronger dollar weakens demand for oil.