The most-active Brent crude contract for October fell 58 cents, or 1.3 per cent, to $43.51 a barrel at 10.55 GMT. The September Brent contract, which is expiring on Friday, fell 57 cents to $43.18 a barrel.
US West Texas Intermediate (WTI) crude futures were down 70 cents, or 1.7 per cent, at $40.57 a barrel.
Both benchmark contracts rose on Wednesday after the US Energy Information Administration (EIA) reported the largest one-week fall in crude stocks since December.
"The recent resurgence of the coronavirus is an ominous sign that the upside is limited in the immediate future,” Tamas Varga of oil brokerage PVM said.
Deaths from Covid-19 have now topped 150,000 in the United States, while Brazil, with the world’s second-worst outbreak, set daily records of confirmed cases and deaths. New infections in Australia hit a record on Thursday. The potential hit to the demand rebound comes just as the Organization of the Petroleum Exporting Countries (Opec) and its allies, together known as Opec+, are set to step up output in August, adding about 1.5 million barrels per day to global supply.
"The easing Opec+ supply restrictions combined with the return of some US production may test the resilience of market sentiment in the coming weeks," Stephen Innes, chief global market strategist at AxiCorp said.