Oil prices fell on Thursday, with U.S. crude dipping below $40 per barrel, retreating from early gains amid a persistent global glut of crude and refined fuel.
Brent crude futures LCOc1 were down 20 cents at $43.94 a barrel by 1411 GMT.
Rising U.S. stockpiles served as the most visible evidence of oversupply in oil markets.
Goldman Sachs said on Thursday there remained a downside risk to oil prices "as storage utilization continues to climb". The bank added that "we don't believe that current prices present an appealing entry point".
"Ultimately the focus will return to the balance of demand relative to supply, and until inventory data provides evidence of a tighter supply, the path of least resistance will be lower," CMC Markets chief market analyst Michael Hewson said.
International benchmark Brent is down nearly 12 percent this month and 24 percent this year, having slumped from as high as $115 in 2014.
Brent has not closed higher for two consecutive days since early October.
U.S. crude CLc1 last traded down 64 cents at $40.11, having touched $39.89, the lowest since August. The contract fell below $40 for the first time since August on Wednesday.
Traders are preparing for another downward turn in prices by March 2016, market data suggests, as what is expected to be an unusually warm winter dents demand just as Iran's resurgent crude exports hit global markets after sanctions end.
U.S. crude inventories rose by 252,000 barrels last week to 487.3 million barrels, close to record highs, according to the Energy Information Administration, highlighting that more oil is being produced than is needed.
Because of the glut, producers are scrambling to offer discounts in an effort to defend market share against their competition.
OPEC's second-biggest producer, Iraq, has sold some crude grades for as little as $30 a barrel, trade sources said, acting as a further drag on futures.