U.S. oil prices sank into a bear market Thursday as a global glut of crude shows little sign of abating.

Prices had settled below the key $50-a-barrel mark for the first time since early April on Wednesday, and they kept falling for most of Thursday. An unexpected increase in U.S. crude inventories, announced earlier this week, combined with high production already coming from the Organization of the Petroleum Exporting Countries, is raising concerns that the market is settling into a long period of oversupply.

Light, sweet crude for September delivery settled down 74 cents, or 1.5%, at $48.45 a barrel on the New York Mercantile Exchange, the lowest settlement since March 31. The U.S. benchmark has posted losses in 17 of the past 21 sessions. That skid has now dropped it into a bear market, down more than 21% from the 2015 high that it hit just a month ago.

Brent, the global benchmark, fell 86 cents, or 1.5%, to $55.27 a barrel on ICE Futures Europe. That is Brent's lowest settlement since April 2.

"We expect the market to remain heavily oversupplied for the next year, keeping prices subdued," said Thomas Pugh, commodities analyst at Capital Economics.

Oil prices rose through the spring on expectations that drilling cutbacks would lead to a decline in U.S. oil production. That hasn't become a reality, and oil production has indeed risen in Saudi Arabia, Iraq and elsewhere.

The U.S. Energy Information Administration said Wednesday weekly oil inventories increased by 2.5 million barrels, while analysts were expecting a fall. Few things changed Thursday, and brokers and analysts said a general sense of bearishness is simply pervasive.

"A lot less rigs being drilled ought to mean production declines, and that hasn't happened so far," said Rusty Braziel, an energy consultant.

Consultancy Energy Aspects estimated that global supplies rose by a substantial 690,000 barrels a day over the month of June, reaching 95.08 million barrels a day. The biggest share of the uplift came from OPEC producers, with Saudi Arabia pumping around 10.6 million barrels a day, a record high, Energy Aspects said.

"By expanding its production to a three-year high, OPEC is ensuring that the oil market remains amply supplied," analysts at Commerzbank said in a report. "No change can be expected here in the near future: OPEC delegates made it clear that they view the weak price as temporary and plan to stick with their strategy of defending market shares."

Gasoline futures settled down 1.55 cents, or 0.8%, at $1.8521 a gallon, their lowest settlement since April 14. Diesel futures fell 1.7 cents, or 1%, to $1.6546 a gallon, the lowest settlement since Jan. 29.

Nicole Friedman and Eric Yep contributed to this article.

Write to Timothy Puko at tim.puko@wsj.com and Georgi Kantchev at georgi.kantchev@wsj.com

Corrections & Amplifications

Oil prices fell to a three-month low. A previous version of this story misstated that oil prices fell to a two-month low. (July 23)

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