Brent crude futures` August front-month contract LCOc1 was down 25 cents at $50.40 a barrel.
On Monday, it climbed $1.48, or 3 percent, to $50.65 a barrel. The contract has risen about 7 percent since Thursday`s settlement, after dropping 10 percent in six previous sessions.
Two opinion polls released on Monday suggested support for Britain staying in the European Union had recovered some ground following the murder of a pro-EU lawmaker last week, although a third survey found backers for a "Brexit" ahead by a whisker.
While concerns over a British exit fade into the background, however briefly, supply issues are back in focus.
Saudi Arabia`s crude oil exports dropped in April despite high production levels, suggesting its battle for market share against U.S. shale drillers may be running its course.
With oil prices up more than 30 percent this year, shale drillers are looking at turning the taps on again and have proved resilient beyond Saudi and OPEC expectations.
"Yet while tentative signs of rising drilling activity have materialized in a rising rig count in recent weeks, many producers are nervous about ramping up drilling operations without seeing a period of price stability," Matt Smith, director of commodity research at ClipperData, said in a note.
Potentially adding to supply, Iran has increased its crude exports capacity at its main terminal on Kharg Island to allow eight tankers to load simultaneously, the oil ministry`s news agency Shana reported on Monday.
Meanwhile, Nigeria`s naira NGN=D1 slumped 30 percent against the dollar on Monday after the country`s currency peg was removed to alleviate the chronic foreign currency shortages choking growth in Africa`s biggest economy and major oil exporter.
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