Brent crude for September fell 15 cents, or 0.2%, to $75.40 a barrel by 0411 GMT while U.S. West Texas Intermediate crude for August was at $74.44 a barrel, down 12 cents, or 0.2%.
The spread of coronavirus variants and unequal access to vaccines threaten the global economic recovery, finance chiefs of the G20 large economies warned on Saturday.
A Reuters tally of new COVID-19 infections shows them rising in 69 countries, with the daily rate pointing upwards since late-June and now hitting 478,000.
"We've not yet seen the impact but at this rate, it will hit demand sooner or later," a Singapore-based oil trader said.
Oil prices slumped last Tuesday after the Organization of the Petroleum Exporting Countries and their allies, a group known as OPEC+, did not reach an agreement to increase output from August. This was because the United Arab Emirates rejected a proposed eight-month extension to OPEC+ output curbs.
"Prices are going to stay volatile for as long as the impasse remains," said Howie Lee, an economist at Singapore's OCBC bank.
"On the surface it looks like it should support prices, but the risk remains that a complete breakdown will result in a price war not dissimilar to last year," he said, but added that the probability of the latter event was low.
The world's top oil exporter Saudi Arabia met full contractual demand for crude oil from five buyers in August, but turned down at least two requests for additional volumes.
Front-month WTI crude futures posted their sixth weekly gain last week after a bullish report from the U.S. Energy Information Administration showed U.S. crude and gasoline stocks fell while gasoline demand reached its highest since 2019.
In response to higher oil prices, U.S. energy firms added oil and natural gas rigs for a second week in a row, data from Baker Hughes showed.