U.S. oil prices closed off session lows on Monday as the dollar rally lost momentum, though gloomy economic prospects in Europe and Asia weighed on crude futures.
The softening comes a week after crude prices hit 2016 highs on the back of a quicker-than-expected rebalancing in physical oil markets.
Crude futures clawed back some ground as the dollar's rally stalled in morning trade.
Still, the dollar has risen about 1.4 percent from June lows, lifted by Brexit worries, concerns over Asia's economy and the prospect of a potential hike in U.S. interest rates. The strength in the dollar makes fuel imports for countries using other currencies more expensive.
Worries that Britain will vote later this month to leave the European Union, sent stocks spiraling lower on Monday, and could erase more of oil's recent gains, traders said.
Brent crude oil futures fell 24 cents at $50.30 per barrel, after striking a session low of $49.61.
U.S. crude settled 19 cents lower, or 0.4 percent, at $48.88 a barrel, having earlier fallen to $48.16.
The Organization of the Petroleum Exporting Countries in a monthly report said its production fell by 100,000 barrels per day (bpd) in May led by output declines in Nigeria. OPEC's report points to a supply deficit of 160,000 bpd in the second half of 2016 if the group keeps pumping at May's rate.
Two straight weeks of rising rig counts in the U.S. increased jitters in the industry about higher production, which could delay the ongoing global rebalancing in the crude market.
Oil traders have already sold out of long positions that have profited from an almost doubling in crude prices since hitting over decade lows earlier this year.
However, continued supply outages that have buoyed prices are keeping some concerns at bay.
On Monday, the Niger Delta Avengers, a militant group that has claimed the bulk of the attacks on the country's oil infrastructure over the past several months, refused to immediately commit to peace talks and said it could reconsider its vow not to take lives.
On Monday, the European Central Bank also said the fall in oil prices over the past two years would add less to global growth than earlier thought and the overall impact could even be negative.
There are also worries about faltering growth in China, largely due to industrial overcapacity and spiraling debts. But some analysts expect oil demand in Asia and especially China to remain strong.
Vehicle sales in China rose 9.8 percent to 2.1 million units in May, the China Association of Automobile Manufacturers said on Monday, in the strongest year-on-year growth since December 2015.
In the first five months of 2016, sales were up 7.0 percent.
"Against the backdrop of low international oil prices, Chinese crude oil demand will remain well supported this year as demand continues to gain traction from stockpiling activities and refining use," energy consultancy FGE said.
"We expect Chinese crude oil imports to grow by 730,000-760,00 bpd this year," it said.