By Christopher Johnson
Benchmark Brent crude was down 30 cents at $75.58 a barrel by 0910 GMT. U.S. light crude was 20 cents lower at $66.16.
The Organization of the Petroleum Exporting Countries and some non-OPEC producers, including Russia, started withholding output in 2017 to reduce a global supply overhang and prices have risen by around 60 percent over the last year.
OPEC and other producers will meet on June 22-23 in Vienna to discuss future production policy.
"Saudi Arabia and Russia have already started to lift production," he said. "Unofficial sources have said that Russia will propose to return production back to the October 2016 (level), i.e. removing the cap altogether over a period of three months."
"The prospect of easing supply curbs from OPEC-led producers continues to be reflected in oil's overall depressed price," Otunuga said.
Rising U.S. stocks partly reflect a surge in U.S. crude production, which has jumped by almost a third in the last two years to a record 10.8 million barrels per day (bpd).
With output in Russia rising back above 11 million bpd in June and Saudi production climbing to more than 10 million bpd, supplies from the top three producers are increasing.
"With rising production from U.S. shale adding to oil's woes and reviving oversupply concerns, further downside could be a possibility in the short to medium term," Otunuga said.