Oil prices fell nearly 2 percent on Thursday, hitting three-month lows, after a fresh stock build at the delivery hub for U.S. crude futures added to concerns that producers were pumping more than needed.
U.S. crude's WTI contract entered a technical bear market at $41.29 a barrel, having fallen at least 20 percent from its 2016 intraday high of $51.67 at that level.
U.S. light crude settled down 78 cents, or 1.86 pct, at $41.14 a barrel, and was last down 84 cents, or 1.98 percent, at $41.09 at 3:23 p.m. ET, after touching a session low of $41.04.
Brent crude oil was down 80 cents, or 1.84 percent, at $42.67 per barrel.
Both crude benchmarks hit new multi-month lows on Wednesday after U.S. government data revealed a surprise rise in crude and gasoline inventories. The build added to an already huge global refined product glut just as slowing economic growth dents the demand outlook.
Following the report, Bart Melek, head of commodities strategy at TD Securities, told CNBC oil could be heading to its 200-day moving average at around $41 per barrel.
Surplus barrels of gasoline have raised concerns about a renewed glut of feedstock crude oil. U.S. Gulf Coast gasoline stocks hit record highs last week for a July month, while East Coast inventories reached all-time peaks, government data showed on Wednesday.
Market intelligence firm Genscape added to the bearish sentiment on Thursday, reporting a build of nearly 328,000 barrels at the Cushing, Oklahoma delivery hub for U.S. crude futures during the week to July 26, traders who saw the data told Reuters.
Oil markets have been dogged by oversupply for the last two years and fell by as much as 70 percent between 2014 and early 2016, when Brent hit the lowest in more than a decade at around $27 per barrel.
Markets have since recovered some ground but oil remains very weak and low refining margins are hurting energy companies.
Energy major Royal Dutch Shell reported a more than 70 percent fall in quarterly profit on Thursday, well below analysts' estimates, as weak oil and gas prices further ate into revenue.
Shell's net income came in at $1 billion in the second quarter, compared with expectations of $2.2 billion and $3.8 billion achieved in the same period last year.
"Lower oil prices continue to be a significant challenge across the business, particularly in the upstream (operations)," said Chief Executive Ben van Beurden.
Mihir Kapadia, CEO at wealth management firm Sun Global Investments, said oil was still being depressed by concerns over a supply glut and waning demand from key international markets.
In China, rail freight volume fell 7.5 percent in the first half of 2016 from the same period a year earlier to 1.58 billion tonnes, the country's top economic planner said on Thursday.
Tamas Varga, lead oil analyst at London brokerage PVM Oil Associates, said record high stocks and oversupply could drive crude prices down into the mid $30 per barrel area before a significant rally.
"The trend is still down and the bears seem to be in control," Varga said, adding: "But we could see an upside correction before driving lower again."
— CNBC's Tom DiChristopher contributed to this report.